COOK pasta as directed on package, decreasing cooking time by 4 min.; drain.
MEANWHILE, heat oil in large nonstick skillet on medium-high heat. Add onions and garlic; cook and stir until onions are tender. Stir in pasta, marinara sauce, ricotta cheese and 1-1/2 cups of the mozzarella cheese; cover. Reduce heat to medium-low; simmer 10 min., stirring occasionally.
SPRINKLE with remaining mozzarella cheese. Remove from heat; cover. Let stand 5 min. or until cheese is melted.