Cut the top 1/4 off the bell peppers and remove the seeds and membranes, place bell peppers in a large baking dish and set aside. Dice and reserve the extra flesh from the tops, discarding the stem.
Heat a large saute pan over medium high heat. Brown the sausage, then remove sausage from pan.
Add the onions, the diced parts of the bell peppers, and mushrooms, season with salt and pepper. Sauté for 10-15 minutes, until they have released their water and water has evaporated from pan.
Once water has evaporated and pan is starting to get dry, add the tomato paste and garlic, stir together until mixed and becoming fragrant, 1-2 minutes.
Add the meat back to the pan, stir in oregano and parmesan cheese. Stir until fully mixed.
Fill bell peppers with sausage mixture and bake for 30 minutes.
After 30 minutes, remove from oven and top peppers with mozzarella cheese, return to the oven for 15-20 minutes, or until peppers are cooked and cheese is browning and bubbly.