Lightly coat the inside of the slow cooker with cooking spray.
Heat a large skillet over medium heat and cook the ground beef, Italian sausage, and diced onion until browned.
Drain any excess grease and stir in the minced garlic. Cook for 30 seconds.
Add the marinara sauce, tomato sauce, Italian seasoning, oregano, garlic powder, onion powder, salt, and black pepper. Simmer for 5 minutes.
In a mixing bowl, combine the ricotta cheese, 1 cup mozzarella cheese, Parmesan cheese, egg, and parsley until smooth.
Spread a thin layer of meat sauce on the bottom of the slow cooker.
Add a layer of broken lasagna noodles.
Spread a layer of the ricotta cheese mixture over the noodles.
Repeat the layers of meat sauce, noodles, and cheese mixture until all ingredients are used.
Finish with the remaining meat sauce and sprinkle the remaining 1 cup mozzarella cheese over the top.
Cover and cook on LOW for 4–5 hours, or until the noodles are tender and the cheese is melted.
Turn off the slow cooker and let the lasagna rest for 15–20 minutes before slicing.
Garnish with chopped parsley and extra Parmesan cheese before serving.