Spray the inside of the slow cooker with non stick cooking spray.
Brown the ground beef in a large skillet on the stove. Drain fat if desired. Add the diced onion and cook for another 3-5 minutes until the onions are soft and translucent.
Place about a third of the spaghetti sauce in the bottom of a slow cooker.
Add half the frozen ravioli in an even layer on top of the sauce. Top with half the meat mixture and half the remaining sauce.
Repeat with a second layer of ravioli, meat and sauce finishing with cheese. Cover and cook on low for 2 ½ to 3 hours.
Notes
Storage
You can store this easy meal in an airtight container (once completely cool) in the refrigerator for up to 4-7 days. Freeze leftovers for up to 3 months or making it ahead of time to be frozen till ready to serve.Thaw overnight in the refrigerator and then reheat in an oven-safe dish, covered with aluminum foil to avoid any burning.