Autumn is a great time for those classic comfort foods that we all grew up loving (at least on the East Coast of the US).
Classic beef stew was something most kids were served on a cold evening after raking leaves all afternoon.
Most vegetarians I know spend a lot of time trying to find non-meat versions of classic dishes, many of which are every bit as delicious as the original meal, but without the cruelty and usually healthier.
Replacing the beef with a cubed seitan (either buy some at the store, or try making this one in advance) is a great way to make this as hearty as any stew out there.
You can do this on the stove on low heat, but I prefer to use a crock pot and cook it slowly.  This mingles the flavors better and is easier to dump everything in and walk away for a few hours.
Start by skinning chopping up the potatoes, yam and turnip, rough 1x1 inch cubes will do great.  I like to boil these for a few minutes just for good measure, but you can skip that step if you're doing the slow cook method. 
Dice the onion and pepper.  Slice the carrots and celery.
Cube the seitan and don't try to drain all the liquid as you go, this helps add flavor to the stew.
Mix all of these ingredients in the crock pot or a large stock pot on the stove on low heat. 
Stir the gravy packets into the water and pour in over the mixture of veggies.  Add whatever spices you find interesting, I listed an easy group in the ingredients here, but there is not any standard set if you look at other stew recipes, get creative.
The gravy will thicken while it cooks, so don't worry that it looks watery just now. 
Let it cook on low for about 45 minutes on the stove, or for however much time you have in the crock pot.  A long, slow cooking time will really blend the flavors well and make it hearty.  If you're short on time, put the crock pot on high for 20 minutes, then reduce to low for another 20 and you should be good to go. 
This also re-heats very well.
Serve it with a thick, crusty bread for dipping and some hot apple cider. 

Ingredients:
4-6 cutlets of seitan
2 potatoes
1 sweet potato or yam
1 turnip
1 onion
1 pepper
2 carrots
2 stalks celery
2 packets of vegetarian brown gravy mix
2-3 cups of water
Seasonings to taste:
salt
pepper
garlic
paprika
crushed red pepper

Horizon's in Philadelphia makes what might be one of the best appetizers ever, vegan or not.
They have a Jamaican barbecued seitan that is unreal.
Their secret is the seitan.
Ray's Wheat Meat is made by an Allentown, PA company and is available in Whole Foods and other retail outlets in the region that are veg-friendly.
I tried to approximate the joy I found at Horizon's, but for far less money.
A container of Ray's runs about 4 bucks at Whole Foods, and I'll be experimenting with other sauces, but my first attempt was with a bottle of whatever-was-on-sale barbecue sauce. 
Lightly coat a frying pan with some olive oil on a medium-high heat.
Break the seitan into rough pieces about bite sized and heat them up until they are hot all the way through.  This should only take a couple of minutes.
Coat each one with the sauce and sear them on each side. 
Serve them hot and enjoy.
The sauce at Horizon's is better.  The smoked chipotle dip they serve with it is unbeatable.  But this low end version of the Horizon's classic is easy and satisfying. 

This recipe starts with seitan.  You can either make you own like this one, or buy some from a store. 
You want to start by finely chopping and onion and a green pepper.  Mince the garlic unless you're using the wonderful pre-minced jar of garlic.   
Heat up a large splash of oil in a heavy skillet (cast iron works great) over a medium-high heat. 
saute the onions, garlic and pepper until the onions are clear.  and starting to get a little brown. 
While the veggies are sauteing, shred the seitan with either a food processor, or just by chopping it finely with a knife. 
Once the veggie mix is ready (slightly browning) mix in the shredded seitan. 
Let this heat up for a few minutes and then add the barbecue sauce.  This is not an exact science, just pour it in from the bottle and mix well.  Once everything looks like it is coated, you've put in enough sauce. 
I've found that the best flavor for this comes from letting the sauce burn on the bottom of the pan and scraping it into the mix.  So let everything cook well and wait a little while between stirs.  It will be cooked pretty quickly, but a little bit of burning this will actually add to the flavor, just try not to go overboard. 
It is done when it looks like sloppy joe, dark red but still moist and as burnt as you like it to taste.
Put it onto your favorite rolls alone or with some lettuce and sliced tomatoes.  Toasting the rolls first makes a nice contrast in textures to the soft, sloppy filling to the sandwich.

Ingredients:
Seitan (I usually shred about 6 or 7 cutlets, not sure how that equals store-bought, but it's about 2 cups or so after shredding)
1 bottle of barbecue sauce (most brands are vegan if they are not the honey or bacon flavored varieties)
One onion
One green pepper
One clove of garlic, minced (or the jarred stuff works great)
Rolls

I found this at VegWeb.com:

Ingredients:
substitute for 2 eggs (mix 1 tablespoon arrowroot powder, 1 tablespoon corn starch, and 4 tablespoons water)
1 tablespoon soy sauce
1 box medium firm silken tofu (350 g)
3/4 cup chopped walnuts
1 packet vegan dried onion soup mix (1.5 oz)
1 teaspoon oil
1 1/2 cups chopped onion
2/4 cup chopped celery
2 cups chopped mushrooms (use portobello mushrooms for a heartier taste)
1 1/2 teaspoon each of oregano and basil
1/2 teaspoon sage
1 1/2 cups bread crumbs (Variations: To make it stick together better, try lowering the bread crumbs to 3/4 cup and adding one of the following: 1 extra box of tofu, 1 cup of instant mashed potato flakes, 1 - 1 1/2 cup of cooked brown rice or 1 cup burger-style crumbles)

Directions:

Mix egg substitute, soy sauce, tofu & onion soup mix together in blender. Add walnuts & blend until smooth.

Saute vegetables until onions are transparent (add other diced veggies if you wish - ie peppers, carrotsetc). Add herbs/spices while vegetables are frying.

Mix blender ingredients, cooked vegetables and bread crumbs together in a large bowl.

Press into a greased loaf pan. (Or, as user giraelei suggests, "Instead of a loaf pan, I usually use a square cake pan and liberally coat the loaf with ketchup so it gets kind of burnt and caramelized. Yum!")

Bake at 350 degrees F. for 1 hour and 15 minutes. Let cool slightly. Turn loaf out and slice.

This has become a standard dish at my parents house when all sorts of eating habits must be catered to. Even the most obstinate meat eaters love it. It is also excellent the next day cold on a sandwich with a bit of mustard.

I constantly get the barrage of questions and comments from people about the impact of a vegetarian diet on a human's health.
Our bodies are designed to break down foods from animal and plant sources.  People will always argue that it means we should be eating an omnivorous diet.
But a world filled with millions of healthy, happy vegetarians will prove that it is quite possible to live and flourish with a completely vegetarian diet.
http://www.medicinenet.com/vegetarian_and_vegan_diet/page2.htm
MedicineNet.com has a nice piece about the idea of a balanced vegetarian diet providing all of the nutrition that a human needs to survive. 
Sure, if you're a vegan who eats nothing but Swedish Fish and Peanut Chews, then of course your body is going to be in terrible condition.  But if you're a meat eater who only consumes burgers, you're going to be in a bad place, too. 
The key is always having a balance, whether you are vegan, vegetarian, pescatarian, omnivore or carnivore.
Variety is the key.