Slow Cooker Ravioli Lasagna

In a large skillet over medium-high heat, cook the ground beef and onion until the beef is browned and the onion is tender, breaking the meat up with a wooden spoon as it cooks; drain any excess grease. Stir in the minced garlic and cook for 1 minute until fragrant.
Transfer the beef mixture to a 6-quart slow cooker. Pour in the entire jar of marinara sauce. Fill the empty jar with 24 fluid ounces of water, swirl it to collect the remaining sauce, and add it to the slow cooker. Stir in the dried basil, salt, and pepper until well combined.
In a medium bowl, mix the ricotta cheese, egg, and 1 cup of the mozzarella cheese until fully blended and smooth.
Spoon half of the meat and sauce mixture from the slow cooker into a separate bowl to make assembly easier. Arrange a single layer of frozen ravioli over the remaining sauce in the slow cooker. Drop half of the ricotta cheese mixture by spoonfuls over the ravioli and gently spread it into an even layer.
Carefully spoon the reserved meat sauce back over the ricotta layer. Top with the remaining frozen ravioli, followed by the remaining ricotta mixture. Sprinkle the top with the final 1 cup of mozzarella cheese and the 1/2 cup of Parmesan cheese.
Cover and cook on LOW for 4 to 5 hours, until the ravioli is tender and the cheese on top is melted and bubbly. Let the lasagna stand for 15 minutes with the slow cooker turned off before serving to allow it to set for cleaner slices.