Spray 5-quart slow cooker with cooking spray.
In 8-inch skillet melt butter over medium heat. Cook onion and garlic in butter, stirring frequently 2 to 3 minutes or until translucent and soft.
In large bowl stir onion mixture and remaining ingredients until well combined.
Place in slow cooker. Cover and cook on Low heat setting 3 hours; stir. Cover and cook on Low heat setting 2 to 3 hours longer or until center of casserole reaches at least 165°F and edges are bubbly. Stir