In a large skillet over medium-high heat, pour in about 2 tablespoons of olive oil. Once shimmering, add one diced medium yellow onion, sauté for 1-2 minutes until softened.

Add 1 pound of lean ground beef, seasoning it generously with salt and pepper. Cook for 5-7 minutes until browned, then stir in 4 minced garlic cloves and cook for an additional 30 seconds.

Spread 1 cup of marinara sauce evenly across the bottom of your slow cooker. Layer 3 uncooked lasagna noodles, followed by 1/3 of your meat mixture, another cup of marinara sauce, and sprinkle with 2/3 cup of shredded mozzarella cheese.

Repeat the layering process two more times with noodles, meat, sauce, and cheese. For the final assembly, top with 3 noodles, 1.5 cups marinara sauce, and remaining shredded mozzarella cheese.

Cover the slow cooker and set it to low heat for about 4 hours, checking for softness around the 3.5-hour mark.

Once done, turn off the slow cooker and let it rest for 20-30 minutes with the lid on.

Serve portions with grated Parmesan on top and garnish with freshly chopped parsley or basil.