Heat the olive oil in a large pot over medium heat. Add the chopped onion and sauté until it becomes translucent (about 5 minutes). Stir in the minced garlic and cook for another minute until fragrant.
Add the crushed tomatoes, oregano, basil, black pepper, and red pepper flakes. Let the sauce simmer on low heat for 20-30 minutes, stirring occasionally.
Preheat your oven to 375°F (190°C).
Spread a thin layer of marinara sauce on the bottom of a 9x13-inch baking dish.
Add a layer of lasagna noodles, followed by half of the ricotta cheese, a third of the marinara sauce, half of the mozzarella cheese, and any optional vegetables.
Repeat the layers, finishing with noodles, marinara sauce, and the remaining mozzarella cheese.
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.
Allow the lasagna to cool for about 10 minutes before slicing to help the layers set for easier serving.