Brown the Beef: Cook the ground beef and onion in a skillet over medium-high heat for about 8 minutes. Add garlic during the last minute, drain the fat, and season with Italian seasoning, salt, and pepper.
Make the Cheese Layer: Mix the softened cream cheese, ricotta, sour cream, and half the Parmesan until smooth and creamy.
Prep the Crock Pot: Spray a 6-quart slow cooker with nonstick cooking spray to prevent sticking.
Start Layering: Spread half a jar of marinara in the slow cooker. Add half the uncooked pasta, the remaining sauce from that jar, half the liquid, and half the beef.
Add the Creamy Layer: Spread the cheese mixture evenly over the beef. Do not stir.
Finish the Layers: Add the remaining pasta, second jar of marinara, remaining liquid, and remaining beef. Press the pasta down so the noodles are submerged.
Slow Cook: Cover and cook on LOW for 2½–3 hours or HIGH for 1½–2 hours. Check the pasta early to prevent overcooking.
Add the Cheese Topping: Gently stir the cooked pasta, then sprinkle mozzarella and the remaining Parmesan on top. Cover and cook for another 10–15 minutes until melted.
Rest and Serve: Turn off the slow cooker and let the pasta rest uncovered for 10 minutes. Garnish with fresh parsley or basil and serve warm.