Heat a large Dutch oven over medium-high heat. Add the ground beef, Italian sausage, and diced yellow onion, breaking up the meat with a wooden spoon as it cooks for 6–7 minutes until deeply browned. Drain any excess fat.

Stir in the minced garlic, Italian seasoning, salt, and tomato paste, cooking for 90 seconds until caramelized and fragrant.

Pour in the 4 cups of broth and the 28-ounce can of crushed San Marzano tomatoes, scraping up the browned bits from the bottom of the pot. Bring to a boil.

Stir the broken lasagna noodle pieces into the boiling broth. Reduce heat to medium-low and simmer uncovered for 10–11 minutes, stirring every 2 minutes so the noodles don't stick, until al dente.

While the noodles simmer, stir together the whole-milk ricotta, half of the shredded mozzarella, grated Parmesan, chopped basil, garlic powder, and black pepper in a small bowl.

Turn the stove heat to low and stir the 1/2 cup heavy cream into the soup. Ladle the hot lasagna soup into warm bowls and crown each serving with a generous scoop of the Three-Cheese Ricotta mixture and the remaining mozzarella so it melts into stretchy ribbons.