Heat olive oil in a large skillet over medium-high heat. Add the onion and cook for 1-2 minutes, stirring, until it begins to soften.
Add the ground beef (or turkey) to the skillet and season with salt and pepper. Cook until browned and cooked through, breaking up the meat into small pieces as it cooks.
Add the garlic and cook for 30 seconds, stirring. Remove pan from the heat.
Spread 1 cup of the marinara sauce in the bottom of the slow cooker insert. Make a single layer of noodles, breaking them up as needed. Top with 1/3 of the meat, 1 cup of marinara sauce, and 2/3 cup shredded mozzarella.
Make a second layer with noodles, half of the remaining meat, another 1 cup of sauce, and 2/3 cup mozzarella.
Make a third layer with noodles, the remaining meat, 1 cup sauce and 2/3 cup mozzarella.
Top with one more layer of noodles, 1 ½ cups of sauce, and the remaining 1/2 cup mozzarella
Cook in your slow cooker on low heat for about 4 hours, until noodles are soft. Turn off heat and let stand, covered, for 20-30 minutes before serving. Open lid carefully to keep any condensation on the lid from dripping on the lasagna. Serve with grated Parmesan and fresh parsley or basil, if desired.