Preheat oven to 425 degrees. Spray a square baking dish with non-stick cooking spray, (just be sure it's large enough to fit four bell peppers standing up). Set aside.
Spray a large nonstick skillet with non-stick cooking spray, and turn heat to medium high. Add mushrooms (or spinach), onions, and garlic to the pan. Add 1/4 cup water and sauté for about 2-4 minutes, stirring constantly. Remove from the heat when the onions become translucent and the water is completely evaporated. Scoop mushrooms and onions into a small bowl and set aside.
Spray the same non-stick skillet with cooking spray to cook your turkey meat. Pour pizza sauce over ground turkey meat, and stir, while breaking up the turkey meat. Add the rest of the seasoning. (Note: You could also add some chopped turkey pepperonis to the mix at this point if you'd like!) Cook until turkey is fully cooked, or no longer pink and remove from heat.
To the cooked turkey, add mushroom/onion mixture and the rest of the ingredients, (except for the cheese for topping). Stir until everything is well combined.
To prepare the bell peppers for stuffing, lay the pepper on it's side, and cut off top, (which includes the seeds and middle).
Once the peppers are clean inside, they are ready for stuffing! Place the peppers in prepared baking dish.
Spoon 1 heaping cup of the meat mixture to stuff into each pepper. (I had a bit more than one cup for each pepper.)
Place cheese on top of each pepper, (I used slices, but if you are using shredded, use 2 tbs on each). Follow with pepperoni's if you choose to use them.
Bake in preheated oven for 30-35 minutes, or until cheese is melted and lightly golden brown. Enjoy warm, (or store in the fridge for up to one week, or freeze for later!)