Cut off tops of bell peppers; remove seeds and membranes, and place bell pepper cups in a lightly greased 8-inch square baking dish. Remove stems from tops, and discard; chop bell pepper tops.
Saute chopped bell pepper and onion in hot oil in a large skillet over medium-high heat about 5 minutes. Remove from heat, and stir in 1 1/2 cups spaghetti sauce and next 5 ingredients. Spoon mixture evenly into bell pepper cups. Top evenly with remaining 1/2 cup spaghetti sauce.
Bake, covered, at 400° for 45 minutes. Uncover and sprinkle evenly with cheese, if desired; bake 5 more minutes or until cheese melts.
Note: For testing purposes only, we used Boca Burger Crumbles for vegetarian burger crumbles.