Chop up the onion, mushroom (if not canned), and bell pepper. You can chop them in big chunks, which is preferable because it adds eye appeal and makes the chili hardier. Or you can chop them more finely, it is completely up to you. Sauté those in a big sauce pan with the olive oil.
Once the onions become somewhat soft and translucent, add in diced tomatoes (if using canned, reserve the liquid, if using fresh, seed the tomatoes). Add in tomato sauce and allow the sauce to clean the bottom of the pan, in order to keep all of the great flavors. Turn the burner down to medium or medium-high and let it simmer for a while, to thicken the sauce.
Add in beans, reserving the liquid, and add in crumbles. Stir well and add seasoning, combining them completely into the chili.
Cook it for longer at this point to thicken it, serve it as is, or add the reserved liquid to thin it a bit, it is all personal preference.