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Onions: In a large skillet, add 1 teaspoon of olive oil over medium heat. Toss in the onions and salt and pepper. Cook for 8-10 minutes or until the onions have softened and darkened a bit. Pour in the balsamic vinegar and let it reduce for 1 more minute. Remove from heat
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Mushrooms: In a large skillet, heat 1-2 teaspoons of olive oil. Add the mushrooms and toss until they have browned and reduced in size. Add salt and pepper and remove from heat
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Burgers: Divide ground beef in 4 balls, flatten them with your hand to form patties
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Sprinkle salt and pepper
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In a large skillet (or grill) heat over medium-high heat. Cook burgers to your desired doneness. Turn them just once. About 3 minutes on each side
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Remove from heat, add the cheese and cover with foil to rest. About 1-2 minutes
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For sauce: mix all the ingredients in a bowl. Test for seasoning and make the necessary adjustments
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Assembly
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Toast the hamburger buns
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Place lettuce on top
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Place the beef and slab some sauce on top
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Add the tomatoes, mushrooms, onions and the top bun
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Enjoy!