Prepare the Meat Sauce. In a large skillet over medium-high heat, cook the ground beef, Italian sausage and diced onion for 6–8 minutes, breaking the meat into small pieces as it browns. Drain excess grease.

Add the garlic and tomato paste. Cook for 1 minute, stirring until fragrant. Stir in the marinara sauce, crushed tomatoes, Italian seasoning, salt, black pepper and red pepper flakes, if using. Reduce the heat and simmer uncovered for 10–15 minutes, stirring occasionally, until thickened. The sauce should be hearty but still have enough moisture to cook the noodles. If unusually thick or dry, add a few tablespoons of water.

Make the Ricotta Filling: In a medium bowl, combine the ricotta, egg, Parmesan cheese, parsley, garlic powder and salt. Stir until smooth and set aside.

Assemble the Lasagna: Lightly coat a 6-quart oval slow cooker with cooking spray. Spread approximately 3/4 cup meat sauce over the bottom. Add a single layer of uncooked lasagna noodles, breaking them as needed to fit.

Spread one-third of the ricotta filling over the noodles. Sprinkle with approximately 3/4 cup mozzarella cheese, followed by another generous layer of meat sauce. Repeat the layers twice more, reserving enough sauce to cover the final layer of noodles.

Finish with a final layer of noodles, the remaining meat sauce, remaining mozzarella and 1/4 cup grated Parmesan. Make sure the top noodles are completely covered with sauce.

Slow Cook: Cover and cook on LOW for 3–4 hours, or until the noodles are tender when pierced with a knife. Begin checking at 3 hours to prevent overcooking.

Rest: Turn off the slow cooker and remove the lid. Allow the lasagna to rest uncovered for 30–40 minutes before slicing so the layers can set.

Serve: Cut the lasagna into portions with a sharp knife. Use a wide, flexible spatula to lift each serving from the slow cooker. Garnish with fresh basil or parsley and serve.