Sheetpan Lasagna

4 T olive oil, divided
1/2 tsp salt, plus more for the pasta water
1 pkg lasagna pasta broken into two-inch pieces
1 clove garlic, chopped
1/2 tsp red pepper flakes
1 five-ounce container baby spinach
1 C ricotta cheese
1 lb spicy Italian sausage, casings removed
1 red onion, diced
4 C jarred marinara sauce
2 1/2 C shredded low moisture mozzarella, divided
1 1/2 C freshly grated parmesan cheese, divided

Preheat the oven to 425 degrees. Oil a straight sided 12 x 17-inch baking sheet with one tablespoon olive oil. Set aside. Heat a medium skillet over medium heat. Add one tablespoon oil, the garlic and pepper flakes to the hot pan. Cook, stirring often until fragrant, about one 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. Add the spinach to 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 return it to medium heat. Add the remaining two tablespoons oil to the pan along with the sausage and cook, undisturbed until the sausage is beginning to brown on the first side, about three 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 five minutes. Add the onion to the pan and cook until the onion is fragrant and beginning to soften, another three minutes. Remove from the heat. 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 four minutes stirring often to avoid clumping. Drain well, reserving 1/2 cup of pasta water. Meanwhile, add the marinara to the pan with the sausage and onion and mix well. Add the pasta water, pasta, one cup mozzarella cheese and 1/2 cup parmesan cheese to the bowl and stir to combine. Spread the mixture on the prepared baking sheet, leaving some tips and corners sticking up on the tray. Dollop the pasta with the ricotta mixture and sprinkle with remaining 1 1/2 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 five minutes before cutting into squares and serving. Serve with extra warm marinera if desired.