Brown the Italian sausage in a skillet, breaking it into crumbles. Drain well.
Spray the slow cooker insert with cooking spray.
Spread a thin layer of sauce in the bottom of the slow cooker.
Layer half the frozen ravioli, half the sausage, sauce, and mozzarella. Repeat layers one more time.
Cover and cook on low for about 3 1/2 hours or high for about 1 hour 45 minutes, until ravioli is tender and cheese is melted.
Let sit for about 10 minutes before serving so the layers scoop cleanly.
Notes
Use frozen ravioli, not fresh, so it holds up in the slow cooker. Brown and drain the sausage first so the casserole does not get greasy. Grease the crockpot insert because melted cheese sticks around the edges. Rest briefly before serving for cleaner scoops.