1. Place the peeled and cubed potatoes in a large pot of salted water. Bring to a boil and cook until tender, about 15-20 minutes. Drain, then return to the pot. Add butter and milk, mashing until smooth and creamy. Season with salt to taste. Cover and keep warm.

2. In a large bowl, mix ground beef, breadcrumbs, diced onion, egg, Worcestershire sauce, garlic powder, onion powder, salt, and pepper. Divide the mixture into four equal portions and shape them into oval patties.

3. Heat a skillet over medium heat and add a bit of oil. Cook the patties for about 4-5 minutes per side until well browned and cooked through. Remove and set aside.

4. In the same skillet, add butter and olive oil. Add the sliced mushrooms and cook until golden brown, about 5-6 minutes. Sprinkle in flour and stir for a minute.

5. Slowly pour in the beef broth while stirring constantly to prevent lumps. Add Worcestershire sauce and season with salt and pepper. Let the gravy simmer for 5-7 minutes until thickened.

6. Return the cooked patties to the skillet with the gravy. Spoon some of the gravy over the patties, cover, and let simmer on low for another 5-7 minutes.

7. Scoop mashed potatoes onto plates, top with Salisbury steak and spoon over gravy. Serve hot.