In a large frying pan over medium-high heat, brown the extra-lean ground beef, breaking it up with a spoon as it cooks. When the beef is nearly cooked through, add the chopped yellow onion and minced garlic. Season with salt and black pepper, then stir to combine. Pour in one jar of tomato sauce and mix well, allowing the flavors to meld for a couple of minutes. Remove from heat.

Spray the inside of your slow cooker generously with nonstick cooking spray to prevent sticking. Spoon a small amount of the meat sauce from Step 1 onto the bottom of the slow cooker to create a thin layer.

Begin assembling the lasagna by layering uncooked lasagna noodles over the meat sauce base, breaking the noodles as needed to fit. Add dollops of ricotta cheese and a sprinkle of shredded mozzarella cheese over the noodles. Spoon more meat sauce from Step 1 on top. Repeat these layers—noodles, ricotta, mozzarella, and meat sauce—until the slow cooker is full or you've used up the ingredients, typically about 3 layers.

Pour the second jar of tomato sauce evenly over the top layer of the lasagna. Sprinkle the remaining mozzarella cheese on top. Gently pour the water around the sides of the lasagna in the slow cooker to help the noodles cook evenly and avoid dryness.

Cover the slow cooker and cook the lasagna on high for 3 hours, or on low for 6 hours, until the noodles are tender and the cheese is bubbly. Once cooked, let the lasagna rest for a few minutes. Garnish generously with chopped fresh parsley or basil before serving.