Preheat the oven to 425 degrees F. Oil a straight sided 12 x 17-inch baking sheet with 1 tablespoon olive oil. Set aside.

Heat a medium skillet over medium heat. Add 1 tablespoon oil along with the garlic and pepper flakes to the hot pan. Cook, stirring often until fragrant, about 1 minute. Add the spinach and the salt and cook, stirring often, until the spinach is almost completely wilted, another minute. Remove the spinach mixture to a strainer and press firmly to remove any excess liquid. Place the spinach in a medium bowl. Using a pair of kitchen scissors, slice up the spinach into chopped pieces. Add the ricotta cheese and stir to combine. Set aside.

Using a paper towel, wipe out the skillet and bring it back to medium heat. Add the remaining 2 tablespoons of oil to the pan along with the sausage and cook, undisturbed until the sausage is beginning to brown on the first side, about 3 minutes. Using a wooden spoon, break apart the sausage into bite sized pieces. Continue to cook, stirring often and breaking apart as needed, until the sausage is lightly browned, another 5 minutes. Add the onion to the pan and cook until the onion is fragrant and beginning to soften, another 3 minutes. Remove from the heat.

Meanwhile, bring a large pot of water to a boil over high heat. Be sure it is a large enough pan to allow the pasta to swim freely. Season generously with salt. Cook the pasta for 4 minutes stirring often to avoid clumping. Drain well, reserving ½ cup of pasta water.

While the pasta is cooking, add the marinara to the pan with the sausage and onion and mix well. Add the pasta water, pasta, 1 cup mozzarella cheese and ½ cup parmesan cheese to the same pan and stir to combine.

Spread the entire mixture on the prepared 12 x 17-inch baking sheet. Dollop the pasta with the ricotta-spinach mixture, and sprinkle with remaining 1 ½ cups mozzarella and 1 cup parmesan cheeses. Bake until the pasta is tender, the sauce is bubbly, and the top is lightly browned, about 30 minutes. Remove from the oven and let rest for 5 minutes before cutting into squares and serving. Serve with extra warm marinara if desired.