Heat oven to 400 F
Heat olive oil in a large pan over medium heat.
Once oil is hot, add onions to pan and cook until translucent, stirring often (2 to 3 minutes)
Add garlic and cook for an additional minute, while continuing to stir
Add Italian sausage, peppers and mushrooms to pan, cook until meat is browned, about 10 minutes
Drain meat mixture and add to a large bowl along with tomato sauce, tomato paste, crushed red peppers, herbs, salt and pepper. Stir until well combined.
Scoop the filling evenly into each of the four bell peppers. Place peppers standing up into a small glass baking dish.
Bake for 40-45 minutes, until peppers are tender and filling is heated through.