Heat a large skillet over medium-low heat and add the olive oil, onion, green peppers and mushrooms. Cook 3-5 minutes until the vegetables are soft. Add the tomato paste and cook for 2 minutes, then add the marinara sauce and stir
In a baking dish, cover the bottom of the baking dish with half of the sauce. Add the frozen manicotti to the dish and cover the top of the manicotti with remaining sauce, pepperoni and mozzarella cheese. Cover with foil and bake for 30-35 minutes
Divide among serving plates and sprinkle with Parmesan cheese if desired. Serve immediately
