Preheat oven to 350°F. Cover a baking sheet with foil.
Heat skillet on the stovetop to medium heat. Add oil and move around so it coats most of the pan. Add onions and sauté until translucent and soft (7-10 min).
While onion is sautéing, deseed the bell peppers by cutting off the tops of each pepper and then removing the seeds and membranes.
Add diced mushrooms and ground turkey to skillet. Break the turkey into tiny pieces as it cooks. Cook until the turkey is no longer pink (~3-4 min). Remove any drippings from the pan.
Add cooked brown rice, tomato sauce, salt, oregano, thyme and minced garlic to the pan. Mix well and let cook together ~3-4 min.
Stuff peppers with meat mixture. Place on baking sheet.
Bake for 15 minutes in oven. Cover with foil and bake 5 more minutes. Serve.