Salisbury Steak with Mushroom Gravy

Ingredients

• 1 ½ pounds 90% lean ground beef
• ½ cup dried breadcrumbs
• 1 large egg, beaten
• 1 tablespoon Worcestershire sauce
• 1 tablespoon soy sauce
• 3/8 cup tomato sauce
• 1 tablespoon dry mustard
• 1 teaspoon granulated garlic
• 1 teaspoon granulated onion
• 3 teaspoons dried oregano
• Kosher salt
• pepper
• Finely chopped parsley, for serving
• 2 tablespoons olive oil
• 4 tablespoons unsalted butter
• ½ cup thinly sliced onion
• 8 ounces sliced mushrooms
• 2 cups beef broth
• 1 tablespoon tomato sauce
• 1 tablespoon cornstarch or 2 tablespoons tapioca flour