Preheat the oven to 425 degrees F. Grease 12 x 17-inch baking sheet with olive oil or cooking spray
Heat a medium skillet over medium heat. Add 1 tbs. oil along with the garlic and pepper flakes. Cook 1 minute. Add the spinach and the salt and cook. Remove spinach to a strainer and press firmly to remove any excess liquid. Add spinach to medium bowl, Add ricotta cheese, stir to combine. Set aside.
Add the remaining 2 tbs. of oil to skillet with the sausage and cook until the sausage is beginning to brown (about 3 mins.) Break apart the sausage into bite-sized pieces. Add the onion to the pan and cook until the onion has softened. Remove from heat.
Bring large pot of water to a boil. Season with salt. Cook pasta for 4 minutes stirring often. Drain well, reserving 1/2 cup of pasta water.
Add the sausage, onion and marinara to a large bowl and mix well. Add the pasta water, pasta, 1 cup mozzarella cheese and 1/2 cup parmesan cheese to the same pan and stir to combine.
Spread mixture on baking sheet. Dollop with the ricotta-spinach mixture, and sprinkle with remaining 1 1/2 cups mozzarella and 1 cup parmesan cheeses. Bake 30 minutes. Remove from the oven and let rest for 5mins before serving.