Heat olive oil in a large pot or Dutch oven over medium heat. Add Italian sausage and cook, breaking it up with a spoon, until browned. Drain off any excess grease.
Add chopped onion to the pot and cook until softened, about 5-7 minutes. Stir in minced garlic and cook for another minute until fragrant.
Pour in the chicken broth and add the can of diced tomatoes (undrained), dried oregano, dried basil, and red pepper flakes (if using). Bring the mixture to a boil, then reduce heat and simmer for 10 minutes.
Add the broken lasagna noodles to the pot. Cook according to package directions, stirring occasionally, until the noodles are al dente. This usually takes about 10-12 minutes.
In a small bowl, whisk together ricotta cheese, mozzarella cheese, and Parmesan cheese. Stir this mixture into the soup until well combined and melted.
Pour in the heavy cream and stir until the soup is creamy. Season with salt and black pepper to taste.
Ladle the soup into bowls. Garnish with fresh chopped parsley before serving.
Notes
This soup can be made ahead of time. Reheat gently on the stovetop. If the soup becomes too thick, add a little extra chicken broth or water.