Cut tops off of bell peppers, if using italian peppers (they are thinner), lay pepper on cutting board the way it is most stable, then slice off the very top of the pepper at a slight angle (to help keep stuffing inside while cooking in the crock pot). Remove seeds and inner membrane.
Cook brown rice and cool slightly (I use one bag of Success boil in bag brown rice- super easy and always cooks up perfectly).
Saute onions in thin layer of wine, or water if you prefer. When starting to become translucent, add mushrooms and saute until mushrooms lose most water and are reduced in size. Add garlic and saute a minute. Add oregano and saute one more minute. Remove from heat.
In a medium bowl, stir together rice, mushroom mixture, cheese, then salt and pepper to taste. Stuff inside of peppers. Pour half of the marinara into the crockpot, carefully lay peppers on top, then carefully pour remaining sauce on each pepper.