01 -
Spray a 5- or 6-quart oval slow cooker with nonstick spray. In the slow cooker, combine melted butter, tomato paste, and salt. Add thinly sliced onions and mushrooms, tossing to coat evenly. Stir in beef broth and mix thoroughly to create the flavor base.
02 -
In a large bowl, gently combine ground beef, panko breadcrumbs, milk, beaten egg, Montreal steak seasoning, and soy sauce. Form approximately 24 meatballs, each about 1 1/2 inches in diameter.
03 -
Arrange meatballs evenly over the vegetable mixture in the slow cooker. Cover and cook on Low heat for 3 to 4 hours, until meatballs reach 165°F internally and vegetables are tender.
04 -
Whisk cornstarch and cold water to form a slurry. Stir it into the slow cooker, cover again, increase heat to High, and cook for 5 to 10 minutes until the gravy thickens to a rich, silky consistency.
05 -
Spoon meatballs and mushroom-onion gravy over prepared creamy butter mashed potatoes. Garnish with chopped fresh Italian parsley before serving.