Gently combine ground beef, breadcrumbs, milk, egg, minced onion, garlic powder, pepper, and salt in a large bowl, mixing just until combined to prevent toughness.

Divide the mixture into 6-8 oval-shaped patties, about ¾-inch thick, ensuring they are not too thick for even cooking.

Heat olive oil in a large skillet over medium-high heat, then sear patties for 3-4 minutes per side until deeply browned, cooking in batches if needed. Remove and set aside after searing.

Reduce heat to medium, add butter to the same skillet, scraping up browned bits, then whisk in flour for 1-2 minutes until a light golden roux forms.

Gradually whisk in beef broth to the roux, ensuring each addition is incorporated to prevent lumps, then bring to a simmer, whisking occasionally until slightly thickened.

Stir in Worcestershire sauce, Dijon mustard, onion powder, and garlic powder for enhanced flavor.

Whisk the cornstarch slurry into the simmering gravy for 1-2 minutes until the desired diner-style thickness and glossy consistency are achieved.

Return seared patties to the gravy, reduce heat to low, cover, and simmer for 10-15 minutes until patties are cooked through, tender, and have absorbed the gravy's flavors.

Taste the gravy and adjust salt and pepper, then serve the Salisbury steak hot with preferred sides.