Prepare Steak: In a large bowl, toss cubed steak with salt and pepper.
Sear Steak: Sear steak pieces in batches in a large skillet over medium-high heat with oil. Transfer seared steak to the slow cooker.
Add Ingredients: Add onion and garlic to the slow cooker with the steak bites. Pour in beef stock and gravy, and top with butter slices.
Cook: Cover the slow cooker and cook on high for 3-4 hours or low for 6-8 hours, stirring occasionally.
Serve over mashed potatoes. Garnish with fresh parsley if desired.