Preheat oven to 375F.
Heat olive oil in a pan over medium heat.
Saute onion until golden brown, add in ground beef and brown it completely.
Add in garlic and stir well.
Add mushrooms and spinach. Mix well and cook until mushrooms are cooked and spinach is wilted.
Add in cooked rice, ½ cup pasta sauce, Italian seasoning and salt. Stir and combine well. Turn the heat off.
Cut the top of the bell peppers, clean out the seeds and set them aside.
Bring a pot of water to boil and steam the bell peppers for almost 5 minutes.
Fill the bell peppers with the filling, top with shredded mozzarella and parsley. Place the top part of the bell peppers on as lids.
Pour the remaining pasta sauce in a baking dish and place the stuffed peppers in the dish.
Cover with an aluminum foil and bake for 30 minutes.
Uncover and bake for another ten to fifteen minutes until the bell peppers are soft to your preference.