In a large skillet over medium heat, heat oil. Add onion and cook until translucent, 3 to 4 minutes. Add garlic and cook until fragrant, 1 minute. Add beef and season with salt, pepper, and Italian seasoning. Cook until no pink remains. Stir in 3 cups of the marinara until combined and cook 2 to 3 minutes more.
In a large bowl, combine ricotta with Parmesan and parsley. Season with salt and pepper and stir until fully incorporated.
Grease the bowl of your slow cooker with a nonstick cooking spray. Using the remaining marinara, spread a thin layer on the bottom of the crockpot. Next, add a layer of noodles (you’ll have to break some to make them fit), a layer of meat mixture, a layer of mozzarella, and a layer of ricotta mixture. Repeat until all ingredients are used up, ending with mozzarella. Cover and cook on low for 4 to 5 hours.
Garnish Crock Pot Lasagna with more Parmesan and parsley and serve Crock Pot Lasagna.