Pour the frozen ravioli into a 6-quart slow cooker.
If using onion, in a small pan, sauté the diced onion in the pan with a little oil until soft. Add minced garlic in during the last 30 seconds of cooking.
When done, in a medium bowl, stir together the crushed tomatoes, tomato paste, water, onion, minced garlic, Italian seasoning, salt and pepper.
Pour sauce over the frozen ravioli and stir to coat all the ravioli with sauce.
Cover and cook on low for about 5 hours (or high for 3 hours). There is no need to stir this while it cooks.
About 10 minutes before serving, sprinkle the ravioli with the shredded mozzarella cheese.
Place the cover back on the slow cooker and let the cheese melt. Then serve!
Notes
You need to cook the onion before adding to the sauce or it will stay crunchy and won't soften up while cooking in the slow cooker.
If you have a smaller family or just don't want leftovers, one bag of ravioli will work. Keep the sauce recipe the same.
I used a mix of beef and cheese ravioli.
Also, you can substitute sauce ingredients for one large jar of store-bought pasta sauce.