01 -
Spray a 5- or 6-quart oval slow cooker with nonstick cooking spray. In the cooker, mix melted butter, tomato paste, and salt. Add sliced onions and mushrooms and toss to coat. Stir in beef broth.
02 -
In a large mixing bowl, combine ground beef, panko breadcrumbs, milk, egg, steak seasoning, and soy sauce. Mix until well combined. Shape into approximately 24 meatballs, each 1 1/2 inches in diameter.
03 -
Arrange meatballs evenly over the vegetable mixture in the slow cooker. Cover and cook on Low heat setting for 3 to 4 hours, or until meatballs reach an internal temperature of 165°F.
04 -
In a small bowl, whisk together cornstarch and cold water. Quickly stir the slurry into the slow cooker. Cover and increase heat to High setting. Cook for an additional 5 to 10 minutes until sauce thickens.
05 -
Serve meatballs and gravy over prepared mashed potatoes. Garnish with chopped parsley.