- Preheat your oven to 375°F. 
- Cook the rigatoni according to package instructions until al dente. Drain and set aside. 
- In a large skillet, heat the olive oil over medium heat. Add the diced onion and sauté until translucent, about 5 minutes. 
- Stir in the minced garlic and cook for an additional minute until fragrant. 
- Add the chopped spinach to the skillet and cook until wilted, about 3-4 minutes. Remove from heat and let cool slightly. 
- In a large bowl, combine the ricotta cheese, 1/2 cup of the mozzarella cheese, 1/4 cup of the Parmesan cheese, egg, oregano, salt, black pepper, and red pepper flakes (if using). Mix well. 
- Stir in the cooked spinach and onion mixture. 
- In a large baking dish, spread a thin layer of marinara sauce on the bottom. 
- Add the cooked rigatoni to the ricotta mixture and stir to combine. 
- Pour the rigatoni mixture into the baking dish and top with the remaining marinara sauce. 
- Sprinkle the remaining mozzarella and Parmesan cheese on top. 
- Cover the baking dish with aluminum foil and bake for 25 minutes. 
- Remove the foil and bake for an additional 15 minutes, or until the cheese is bubbly and golden. 
- Let the baked rigatoni cool for 5-10 minutes before serving.