Old-Fashioned Salisbury Steak

For the Steaks:

• 1 lb lean ground beef
• 1/2 cup breadcrumbs
• 1 large egg, beaten
• 2 tablespoons ketchup
• 1 tablespoon Worcestershire sauce
• 1 teaspoon Dijon mustard
• 1 teaspoon kosher salt
• 1/2 teaspoon black pepper
• 2 tablespoons butter