Combine ground beef, onion, garlic, 1/2 teaspoon salt, and 1/2 teaspoon black pepper in a large nonstick skillet over medium-high. Cook until beef is no longer pink and onions are softened, about 8 minutes. Transfer to a 6-quart slow cooker.
STEP 3
Stir in broth, marinara sauce, diced tomatoes, tomato paste, crushed red pepper, remaining 1 teaspoon salt, and remaining 1/4 teaspoon black pepper until combined.
STEP 4
Cover and cook on LOW until soup is slightly thickened, 3 1/2 to 4 hours, or on HIGH for 2 hours.
STEP 5
Stir lasagna noodles into crockpot until sheets are submerged. Cover and cook on HIGH until pasta is al dente, about 30 minutes.
STEP 6
Stir in spinach the last 5 minutes of cooking.
STEP 7
Serve with a dollop of ricotta and Parmesan cheese. Garnish with crushed red pepper.