Pre-heat oven to 350°F. 

Heat a large skillet over medium-low heat add olive oil, onion, green peppers, and mushrooms, cook until vegetables are soft 3-5 minutes, then add marinara sauce and stir to combine. 

In a 7 x 13 casserole dish cover the bottom of the baking dish with half of the sauce. Add manicotti to the baking dish. Cover the top of the manicotti with remaining sauce, pepperoni, and mozzarella cheese. Cover with foil and bake for 20-25 minutes. 

Divide among serving plates and sprinkle with parmesan cheese if desired. Serve immediately.