Heat olive oil in a large skillet over medium-high heat. Add the diced onion and cook for 1 to 2 minutes, stirring, until it begins to soften and turn translucent.

Add the ground beef and season generously with salt and pepper. Cook until fully browned, breaking the meat into small crumbles. Drain off any excess grease before continuing.

Stir in the minced garlic and cook for 30 seconds until fragrant. Remove the pan from heat.

Spread 1 cup of marinara sauce across the bottom of a 5 to 6 quart slow cooker insert to prevent sticking. Lay a single layer of uncooked noodles over the sauce, breaking them as needed to fit. Top with one-third of the meat mixture, 1 cup of sauce, and two-thirds cup of shredded mozzarella.

Add the second layer: noodles, then half of the remaining meat, 1 cup of sauce, and two-thirds cup of mozzarella.

Add the third layer: noodles, then the rest of the meat, 1 cup of sauce, and two-thirds cup of mozzarella.

Finish with one final layer of noodles. Pour 1.5 cups of sauce evenly over the top and sprinkle the remaining half cup of mozzarella across the surface.

Place the lid on the slow cooker and cook on LOW for about 4 hours, until the noodles are completely tender when pierced with a fork. Do not cook on high.

Turn off the slow cooker and let the lasagna rest with the lid on for 20 to 30 minutes before serving. When lifting the lid, tilt it away from you so condensation drips away from the lasagna rather than back onto the top layer.

Scoop and serve with grated Parmesan and fresh parsley or basil if desired.