Best Salisbury Steak Recipe with Mushroom-Onion Gravy

Name: Best Salisbury Steak Recipe with Mushroom-Onion Gravy
Prep time: 15 min
Cook time: 30 min
Total time: 45 min
Cuisine: American
Ingredients: 1 pound ground beef; 1/4 cup breadcrumbs; 1 large egg; 2 teaspoons Worcestershire sauce; 1 teaspoon onion powder; 1/2 teaspoon garlic salt; 1/2 teaspoon mustard powder; 1/4 teaspoon kosher salt; lots fresh ground black pepper (, to taste); 1 Tablespoon cooking oil (, for searing beef patties); 8 ounces mushrooms (, sliced); 1 medium onion (, sliced); 3 Tablespoons butter; 2 cloves garlic (, minced or crushed); 1/4-1/2 cup flour (, depending on how thick you like your gravy); 3 cups broth ((beef, chicken or vegetable)); kosher salt (, to taste); black pepper (, to taste)