Heat a large pot or Dutch oven over medium-high heat. Add olive oil, onion, and Italian sausage. Cook, breaking apart until Italian sausage is no longer pink, about 6-7 minutes. Drain excess grease.
Add garlic, Italian seasoning, and red pepper flakes and sauté for 1 minute.
Add crushed tomatoes and broth. Bring to a simmer.
Add the broken-up lasagna noodles and simmer for about 8-10 minutes. Check the directions on your package of noodles and cook until al-dente.
Add chopped spinach, stirring and allowing to wilt for about 30 seconds.
Serve and top with ricotta, mozzarella, and parmesan cheese.
Notes
If you want to add more veggies to the soup, try mushrooms, bell peppers, and/or zucchini. Add the veggies with the garlic and sauté for a minute, then cook the rest of the way in the simmering broth. Feel free to leave out the Italian sausage if want to make it vegetarian.