|Bison Burger with Mushrooms and Spring Onions
6 tablespoons mayonnaise
2 tablespoons freshly grated horseradish
2 tablespoons olive oil
2 medium spring onions, dark green ends trimmed off, white and pale green portions sliced into 1/4-inch-thick rounds (about 1 cup)
2 teaspoons kosher salt
1 teaspoon freshly ground black pepper
8 ounces mixed wild mushrooms, trimmed and quartered if large
1 large egg
2 teaspoons Worcestershire sauce
1 medium garlic clove, minced
1 pound ground bison meat
3 kaiser rolls, cut in half horizontally and toasted
Heat the broiler to high and arrange a rack in the middle. Place a wire rack inside a baking sheet and set aside. Mix together the mayonnaise and horseradish in a small bowl until evenly combined; set aside.
Heat 1 tablespoon of the oil in a medium frying pan over medium heat. When shimmering, add onion, season with 1/2 teaspoon of the salt and 1/4 teaspoon of the pepper. Cook until soft and browned, about 5 minutes, then transfer to a plate. Return the pan to heat, add remaining 1 tablespoon oil, and heat over high heat until smoking. Add mushrooms and cook, stirring rarely, until golden brown, about 5 minutes. Season with 1/2 teaspoon of the salt and 1/4 teaspoon of the pepper, remove from heat, and set aside.
Mix together egg, Worcestershire, garlic, remaining 1 teaspoon salt, and remaining 1/2 teaspoon pepper in a large bowl until egg is broken up. Add bison meat and mix until thoroughly combined.
Shape meat mixture into 3 patties (4 inches wide, about 1/2 cup each) and place on the rack inside the baking sheet. Broil until browned and springy to the touch, about 10 minutes for medium.
To serve, divide horseradish mayonnaise between bottom bun halves and spread evenly. Top with burgers, divide onions and mushrooms evenly between burgers, and close with top bun halves.