Preheat oven to 350 degrees.
Heat a large pot of salted water to a strong boil. Add the pasta and cook at the rolling boil uncovered until the pasta is al dente-edible but still a bit firm. Drain pasta.
While waiting for the pasta water to come to a boil, chop your onion. Add 1 tbsp olive oil to a large saute pan over medium-high heat. Cook down onions for 3 minutes. Add garlic and crushed red pepper and cook for another 2 minutes. Add ground Italian sausage and cook until browned well, breaking up the meat into small chunks. Add fresh chopped spinach and stir till slightly wilted.
Add tomato sauce to the meat mixture and bring to a simmer. Taste and season with salt and pepper if needed.
Spread a thin layer of the sauce in the bottom of a 9x13" casserole pan, then dot the surface with half of the ricotta cheese. Mix a couple ladles of the sauce into the past and stir well. Add the pasta to the casserole dish.
Dot the top of the pasta with the remaining half of ricotta cheese and pour the rest of the sauce overtop. Sprinkle it with mozzarella cheese and finish with parmesan cheese.
Bake uncovered in the oven at 350 degrees until the top is lightly browned and the cheese is melted, usually 20-25 minutes. Let stand for 5 minutes before serving.