Heat the olive oil in a large skillet over medium heat.
Add the lean ground beef and cook, breaking it apart with a spoon, until browned and no longer pink, about 5 to 7 minutes.
Stir in the diced yellow onion and cook until softened, about 3 to 4 minutes.
Add the minced garlic and cook for 30 seconds, just until fragrant, then remove the skillet from the heat.
Transfer the cooked beef, onion, and garlic mixture to the slow cooker.
Sprinkle in the dried oregano, dried basil, dried thyme, salt, black pepper, and red pepper flakes if using, and stir to combine.
Pour the low-sodium beef broth, crushed tomatoes, and diced tomatoes with their juices into the slow cooker.
Stir in the tomato paste until it is fully dissolved into the liquid.
Cover the slow cooker with the lid and cook on LOW for 4 hours, allowing the flavors to develop.
About 30 minutes before serving, stir in the broken lasagna noodles, making sure they are submerged in the soup.
Cover again and continue cooking on LOW for 20 to 30 minutes, or until the lasagna noodles are tender but not mushy.
While the noodles cook, in a small bowl combine the ricotta cheese, shredded mozzarella cheese, and grated Parmesan cheese, stirring until well mixed.
Once the noodles are tender, taste the soup and adjust the seasoning with a little more salt or black pepper if needed.
Ladle the hot lasagna soup into bowls.
Top each bowl with a generous spoonful of the ricotta, mozzarella, and Parmesan cheese mixture.
Sprinkle chopped fresh parsley over the top for garnish if using, and serve warm.