Classic Homestyle Salisbury Steak with Creamy Garlic Infused Mashed Potatoes

1. **For the Salisbury Steak:**
- In a large bowl, mix together ground beef, breadcrumbs, chopped onion, egg, salt, pepper, ketchup, Worcestershire sauce, and mustard powder. Form into 4-6 patties.
- In a skillet over medium heat, cook patties on both sides until browned and cooked through, about 4 minutes per side. Remove and set aside.
2. **For the Gravy:**