Classic lasagna is time-consuming to make, but all those same ingredients can be made into soup in a fraction of the time! A dollop of ricotta cheese served on top gives this dish the authentic feel of the original. Smarts: Lasagna noodles are broken up and stirred into the soup at the end of cooking. Watch them closely or boil them separately to keep them from over-cooking and getting soggy.
Heat a Dutch oven with oil over medium heat. Stir in onions, garlic, and mushrooms and cook for 2 to 3 minutes.
Add sausage and saute until sausage is breaking apart and nearly cooked through, 4 to 6 minutes.
Stir in tomato paste, oregano, basil, Italian seasoning, and red pepper flakes and saute until fragrant, ~1 minute more.
Pour stock and tomatoes (including liquid) over sausage and aromatics. Add bay leaves. Bring to a simmer.
Stir lasagna noodles into soup and cover.
Simmer soup just until lasagna noodles are al dente, tender but still with some bite. This should take 10 to 15 minutes, but watch closely to prevent over-cooking the noodles. In the last minute of cooking, stir in spinach leaves.
Taste soup and season with some salt and pepper.
Divide soup between serving bowls. Top each bowl with a spoonful of ricotta cheese and some Parmesan cheese (if using). Enjoy!