In a skillet over medium heat, cook ground beef or sausage with diced onion until browned. Drain excess grease.
- Add minced garlic and cook for 1 more minute.
- Stir in marinara sauce and Italian seasoning. Simmer for 5 minutes.
- Spray the inside of your slow cooker with nonstick spray.
- Spread a thin layer of meat sauce on the bottom of the slow cooker.
- Add a single layer of frozen ravioli over the sauce.
- Spread a layer of meat sauce over the ravioli.
- Sprinkle mozzarella and Parmesan cheese over the sauce. Add small dollops of ricotta if using.
- Repeat the layers until all ingredients are used, ending with sauce and cheese on top.
- Cover and cook on low for 4–6 hours or high for 2–3 hours, until the ravioli is tender and cheese is melted.
- Garnish with chopped fresh parsley before serving.