Heat a large, oven-proof soup pot over medium heat. Add the sausage and beef and brown in batches until cooked through. Season with salt, pepper, crushed red pepper, and a shake of garlic pepper if you like. Transfer to a paper towel-lined plate. Drain off all but 1 tablespoon of fat in the pot.
Sauté the onions and mushrooms:
Add the onion to the pot and sauté for 5 minutes. Add the mushrooms and cook for 8–10 minutes. Add garlic and cook for 1 minute. Add salt, pepper, and a pinch of crushed red pepper.
Simmer the tomato sauce:
Return the meat to the pot. Pour in the tomato sauce and water. Bring to a boil. Reduce heat and simmer 20 minutes. Taste and season.
Cook the pasta:
Bring the pot back to a boil. Add the broken lasagna and stir regularly. Boil for 15–20 minutes or until lasagna is al dente. Stir in spinach. Cook for 5 minutes until wilted. Turn off the heat and turn on the broiler.
Make the cheese mixture:
Combine ricotta, half the mozzarella, and the lemon juice in a bowl. Add salt and pepper.
Broil the cheese:
Spoon the cheese mixture onto the soup, covering the entire top of the soup. Sprinkle remaining mozzarella on top. Drizzle with a few teaspoons of extra virgin olive oil.
Place under the broiler and cook until bubbly and golden brown on top.