Heat oil in large skillet on medium heat. Add mushrooms and peppers; cook and stir 3 min. Remove from heat. Add spinach; stir until wilted. Stir in tomato sauce. Mix egg, ricotta, 1-1/2 cups shredded cheese and 2 tablespoons Parmesan until blended. Spoon 1-1/2 cups sauce into slow cooker; top with layers of half each of the noodles (broken to fit) and ricotta mixture. Cover with 2 cups sauce. Top with remaining noodles (broken to fit), ricotta mixture and sauce. Cover with lid. Cook on Low 4 to 5 hours (or on High 2 to 3 hours) or until liquid is absorbed. Sprinkle with remaining cheeses; let stand, covered, 10 min. or until shredded cheese is melted. Tips: Dried pasta will last almost indefinitely if stored in an airtight glass or plastic container in a cool dark place. Dried whole wheat pasta can turn rancid, however, so should be used within the recommended time listed on the package. Variations: Serve with smart sides, such as a crisp mixed green salad or your favorite steamed vegetable, to balance out the meal.