Heat oil in a medium skillet, add onion and bell pepper and cook until softened, about 5 minutes, stirring often. Add tomatoes and simmer, uncovered, until slightly thickened, 5 to 7 minutes, stirring occasionally.
Meanwhile, cook pasta according to package directions; drain and reserve.
Mix together ricotta cheese and spinach; reserve.
Preheat oven to 350° F. Toss pasta with sauce and season to taste with salt and pepper. Place in an 11-1/2 by 8-inch baking dish. Drop spoonfuls of ricotta mixture onto pasta and spread gently to make a thin layer. Cover with foil and bake in center of oven for 30 minutes.
Remove from oven, top with mozzarella and continue to bake, uncovered, until cheese is melted and mixture is hot, 5 to 8 minutes.