Spray 5- to 6-quart slow cooker with cooking spray. In Dutch oven or 12-inch skillet, heat oil over medium heat until hot. Cook onion and garlic in oil about 4 minutes, stirring occasionally, until onion is tender. Stir in pasta sauce, tomato sauce and Italian seasoning.
Place 1 cup of the sauce mixture in slow cooker. Add 1 package frozen ravioli; top with 1 cup of the cheese. Top with remaining package of ravioli and 1 cup cheese. Pour remaining sauce mixture over top.
Cover; cook on Low heat setting 5 1/2 to 6 1/2 hours or until hot. Sprinkle with parsley before serving.