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