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1
In a small bowl, mix mayonnaise, ketchup, and cayenne.
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2
Stir together until totally combined.
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3
Set aside.
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4
(This is the spicy fry sauce.)
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5
Melt butter in a large skillet.
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6
Add onions and cook over medium heat for five minutes, stirring frequently.
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7
Add mushrooms and toss around, then add wine (if using), Worcestershire, salt and pepper.
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8
Cook for several minute over medium heat, or until all liquid has evaporated.
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9
Transfer mushroom mixture to a separate bowl, scraping all contents from the skillet.
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10
Mix ground beef, heavy cream, Worcestershire, and salt and pepper in a bowl.
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11
Use your hands to thoroughly mix ingredients.
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12
Form 1/4 to 1/3 cup of the meat mixture into patties, making an indentation with your thumb to keep the patties from plumping too much when they cook.
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13
In the same skillet as the one used to cook the mushrooms, melt 1 tablespoon butter over medium to medium-high heat.
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14
Add four patties at a time, indented side face up.
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15
Cook for 4 minutes, then flip.
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16
Spoon generous portion of mushroom mixture on each patty, then top with one or two squares of Swiss, depending on your preference.
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17
Place lid on skillet and cook for an additional 2 to 3 minutes, until burgers are cooked through and cheese is melted.
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18
Remove to a place and keep warm while you repeat with the rest of the meat mixture.
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19
Toast halved buns under the oven broiler.
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20
Spread generously with spicy fry sauce.
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21
Place patties between buns and serve immediately with fried onion strings.
