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1
Heat the broiler to high and arrange a rack in the middle.
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2
Place a wire rack inside a baking sheet and set aside.
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3
Mix together the mayonnaise and horseradish in a small bowl until evenly combined; set aside.
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4
Heat 1 tablespoon of the oil in a medium frying pan over medium heat.
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5
When shimmering, add onion, season with 1/2 teaspoon of the salt and 1/4 teaspoon of the pepper.
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6
Cook until soft and browned, about 5 minutes, then transfer to a plate.
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7
Return the pan to heat, add remaining 1 tablespoon oil, and heat over high until smoking.
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8
Add mushrooms and cook, stirring rarely, until golden brown, about 5 minutes.
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9
Season with 1/2 teaspoon of the salt and 1/4 teaspoon of the pepper, remove from heat, and set aside.
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10
Mix together egg, Worcestershire, garlic, remaining 1 teaspoon salt, and remaining 1/2 teaspoon pepper in a large bowl until egg is broken up.
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11
Add bison meat and mix until thoroughly combined.
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12
Shape meat mixture into 3 patties (4 inches wide, about 1/2 cup each) and place on the rack inside the baking sheet.
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13
Broil until browned and springy to the touch, about 10 minutes for medium.
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14
To serve, divide horseradish mayonnaise between bottom bun halves and spread evenly.
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15
Top with burgers, divide onions and mushrooms evenly between burgers, and close with top bun halves.
