Heat olive oil in a large stockpot or Dutch oven over medium heat. Add the chopped onion and minced garlic. Sauté for 2–3 minutes until the onion is translucent and fragrant.
Add the ground beef and Italian sausage. Cook, breaking up the meat as it browns, until fully cooked through. Drain any excess fat.
Add the marinara sauce, diced tomatoes with their liquid, chicken broth, Italian seasoning, red pepper flakes (if using), salt, and pepper. Stir to combine and bring to a rolling boil.
Add the broken lasagna noodles. Reduce to a steady simmer and cook for approximately 15 minutes, stirring frequently to prevent sticking, until the noodles are tender.
Add the spinach and stir until wilted, about 1–2 minutes. Taste and adjust seasoning as needed.
Serve immediately, topped with a generous dollop of ricotta or cottage cheese if desired.