Spray a 6 quart Crock-Pot with nonstick cooking spray. Set aside.
Cook the sausage in a large skillet, over medium heat, breaking it up into small pieces with a spoon, until sausage is cooked through. Remove cooked sausage to a paper towel-lined plate and set aside. Remove all but 2 Tablespoons of the fat from the sausage and discard. Over medium heat, add onion and chopped garlic to the remaining fat in the skillet. Cook until veggies are softened, about 5-8 minutes. Remove from heat and stir in cooked sausage. Set aside.
In a mixing bowl, mix together ricotta, 1 cup of Parmesan, 2 cups mozzarella, fresh basil, 1 teaspoon salt, 1 teaspoon black pepper, and egg.
Spread 1 cup of prepared marinara sauce in the bottom of the crockpot. Layer 4 uncooked pasta noodles, breaking them up as needed to fit, evenly over bottom of the crockpot on top of the sauce. Spread 1/3 of the ricotta mixture, 1/3 of the sausage and onion mixture, 1 cup of sauce. Repeat layers 2 more times.
For the final top layer, arrange 4 more noodles, 1 cup of sauce and the remaining 1 cup Parmesan and 2 cups mozzarella cheese. Cover and cook until on low until lasagna is heated through, about 4 hours.
Turn off heat and let the lasagna cool for 20-30 minutes and serve.