Trim, core and blanch peppers; reserve. Cook and stir Beef and Onion in non-stick skillet until Beef is browned and Onion is soft. Stir in tomato sauce, Mushrooms, herbs, salt and pepper. Stuff Beef mixture in peppers. Place peppers in casserole dish. Pour in enough boiling water to cover bottom of casserole dish. Bake in 350 °F. Oven until peppers are tender, about 20 minutes.