Preheat oven to 425°F.

Bring a large pot of salted water to a boil. Break the lasagna noodles into 2-inch pieces by hand as you add them to the pot. Boil for 4 minutes only (pasta will be undercooked). Reserve ½ cup pasta water before draining. Strain pasta in colander and set aside.

In a large skillet, heat 1 tablespoon olive oil over medium heat. Add chopped garlic and crushed red pepper flakes, sauté briefly. Add chopped spinach and cook until wilted. Press the cooked spinach through a strainer to remove as much liquid as possible. Stir the drained spinach into the ricotta cheese and set aside.

In the same skillet, heat remaining 2 tablespoons olive oil. Brown the Italian sausage, breaking it into smaller pieces as it cooks. Add chopped onion and cook until softened.

Off the heat, add the partially cooked pasta back to the large pot along with reserved pasta water, cooked sausage mixture, marinara sauce, 1 cup mozzarella cheese, and ½ cup Parmesan cheese. Stir to combine.

Scrape the pasta mixture onto the sheet pan, spreading evenly but not pushing too close to edges. Dollop the spinach ricotta mixture on top of the pasta.

Sprinkle with remaining 1½ cups mozzarella cheese and 1 cup Parmesan cheese.

Bake for about 30 minutes, until the top is lightly browned and crispy. Let cool for 5-10 minutes before serving.