Brown the meat: In a skillet over medium heat, cook the ground beef or sausage with the chopped onion until no pink remains and the onion is translucent. Add the garlic at the end and cook until fragrant. Drain excess fat.

Build the sauce: Stir in crushed tomatoes, tomato paste, oregano, basil, red pepper flakes, salt, and pepper. Let the mixture simmer for 3–5 minutes so the flavors marry.

Mix the ricotta layer: In a bowl, combine ricotta, egg, half the Parmesan, a pinch of salt, and chopped fresh basil or spinach.

Assemble the lasagna in the slow cooker: Spoon a layer of sauce into the bottom. Fit lasagna noodles to cover the sauce in a single layer. Spread one-third of the ricotta mixture over the noodles, sprinkle mozzarella, then add a layer of meat sauce. Repeat two more times, finishing with sauce and a final sprinkle of mozzarella and Parmesan.

Cover and cook on low for 3–4 hours or on high for 2–3 hours. The noodles should be tender and cheese should be bubbling.

Turn off the slow cooker and let the lasagna sit, covered, for 10–15 minutes. Garnish with fresh basil and serve warm.