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1
Mix ground beef, Worcestershire sauce, Dijon mustard, salt and pepper thoroughly. Divide mixture and form into two, oval patties. I like to make thinner patties, but shape them so that when they are fully cooked, they cover almost the whole bottom piece of rye bread as this is a sandwich in which each bite should have all the elements. Refrigerate the patties for about an hour or so.
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2
In a large frying pan, melt one tablespoon of butter over medium high heat and add the mushrooms. Make sure all the of the mushrooms get coated with butter, but then leave them untouched for 3-4 minutes so that one side gets some nice color before you stir them around so the other side can get some color.
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3
Remove the mushrooms from the pan. Add 2 tablespoons butter and the onion to the pan and cook slowly over medium to low heat for 10-15 minutes until the onions are soft and somewhat caramelized. Remove the onions from the pan and keep them warm with the mushrooms.
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4
Increase the heat to medium high and fry the burgers to desired doneness. I like mine medium rare, which typically takes 3-4 minutes per side to achieve.
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5
While the burgers are cooking, put a second skillet over medium heat and butter one side of each piece of rye bread. Put the bread buttered side down on the hot skillet until you see some nice color. On two of the slices of bread, place two ounces of Gruyere cheese.
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6
When the burgers are done and the bread nicely browned, assemble your burgers: Bottom piece of bread with cheese, beef patty, onions, mushrooms, optional mayonnaise and top piece of bread. I like to give them a firm press and then slice each in half before serving with a pickle.
