Ingredients

For the burger: 

• 1 pound 80/20 ground beef 
• 1/4 cup finely diced yellow onion
• 2 cloves garlic, minced 
• 1 large egg 
• ¼ cup panko breadcrumbs
• 2 tablespoons water 
• 1 tablespoon finely chopped parsley 
• 1 tablespoon finely chopped sage 
• 2 teaspoons Kosher salt 
• 1 teaspoon freshly ground black pepper 
• 2 tablespoons vegetable oil 

For the gravy: 

• 3 tablespoons unsalted butter 
• ¼ cup finely diced yellow onion 
• 1 clove garlic, minced 
• 8 ounces mixed mushrooms, stems removed and thinly sliced 
• 2 tablespoons all-purpose flour
• 1 tablespoon Dijon mustard
• 1 tablespoon Worcestershire sauce
• 1 cup beef stock 
• 1 tablespoons finely chopped parsley
• 1 tablespoon finely chopped sage
• Kosher salt, to taste
• Freshly ground black pepper, to taste 
• ½ cup grated gruyere or fontina cheese
• 4 brioche burger buns

Directions

In a large bowl mix together beef, onions, garlic, egg, breadcrumbs, water, herbs, salt and pepper until evenly combined. Form the meat into 4 patties and set aside. 

In a large cast iron skillet, heat the vegetable oil until hot. Add in the burger patties, in batches if needed to not overcrowd, and cook 4-5 minutes per side or until deeply caramelized. Remove to a pan and pour out any excess grease. 

In the same pan, melt the butter over medium-high heat. Add in the onion, garlic and mushrooms and saute for about 8-10 minutes or until the mushrooms have cooked down and are caramelized. Stir in the flour and cook 1-2 minutes or until the flour is toasted and evenly coats the mushrooms. Stir in the mustard, Worcestershire sauce and stock until evenly combined. Bring the mixture to a simmer and then stir in the parsley, sage and salt and pepper to taste. Nestle the burgers back into the sauce and then continue to simmer the sauce until it thickens and coats the back of a spoon. Reduce the heat to low, divide the cheese evenly over the top of the burgers and then cover the pan until the cheese is completely melted. Add the burgers to the burger buns and top with the mushroom gravy. Serve immediately.  

Makes 4 burgers