Say hello to the ultimate vegetarian lasagna. Three hearty layers of pasta, marinara, ricotta, mozzarella, and garlicky spinach guarantees satisfaction.
Soak the lasagna noodles with hot water in a baking dish for about 20 minutes.
Prepare the aromatics and cheese: dice onion, mince garlic cloves, grate mozzarella on the large holes of a box grater, and finely grate Parmesan cheese.
Sauté onions and garlic in olive oil until softened. Add spinach handfuls at a time to the pot and stir until wilted.
Season spinach with salt, black pepper, and red pepper flakes, if using. Remove from the heat and let cool for 5 minutes.
Make the filling by combining Parmesan cheese, ricotta cheese, salt, and black pepper.
Drain noodles. Spread marinara on the baking dish. Layer in lasagna, ricotta, spinach, mozzarella, and marinara. Repeat twice. Sprinkle Parmesan last.
Bake at 400°F uncovered until the cheeses are melted and lightly browned, about 30 minutes. Cool for at least 15 minutes before serving.