– 2 pounds lean ground beef – 1 large egg beaten – 1/2 cup panko breadcrumbs – 3 tablespoons ketchup – 1 tablespoon dijon mustard – 1 tablespoon Worcestershire sauce – 3/4 teaspoon black pepper – 1/2 teaspoon kosher salt – 1 crushed beef bouillon cube For the gravy: – 1/2 cup unsalted butter – 1 small onion thinly sliced – 8 ounces baby bella mushrooms sliced – 2 cloves garlic minced – 1/2 cup all-purpose flour – 4 cups beef stock – 2 tablespoons Worcestershire sauce – 1 tablespoon dijon mustard – 1/2 teaspoon black pepper – kosher salt to taste – fresh chopped parsley for garnish