Divide olive oil between two large skillets and heat over medium high heat. Add mushrooms to one and cook until all the moisture has been released and the mushrooms are nicely browned. While mushrooms are cooking, add the peppers and onions to the second skillet and cook until softened, about 5 minutes. Combine vegetables with marinara sauce and set aside.
Combine ricotta and eggs and mix well; toss with cooked pasta. Spray a 13 x 9 inch oven safe baking dish with cooking spray. Layer half of the pasta and ricotta on the bottom and top with half of the vegetable mixture. Layer with ⅓ of the mozzarella cheese. Spread remaining pasta and ricotta over the top, cover with remaining vegetables, and top with remaining ⅔ of the mozzarella. cover with foil and bake until cooked through and bubbly, about 45 minutes.
Recipe by Cooking With MicheleĀ® at https://cookingwithmichele.com/2012/10/cooking-for-others-vegetarian-baked-ziti/