Ingredients

• 1 medium onion, chopped
• 10 ounces portobello mushrooms, trimmed and quartered
• 2 tablespoons plus 2 teaspoons olive oil, divided
• 1 pound ground buffalo
• 2 celery ribs
• 1/2 Granny Smith apple, cored (left unpeeled)
• 1 tablespoons mayonnaise
• 1 1/2 teaspoons cider vinegar
• 1 teaspoon olive oil
• 1 teaspoon whole-grain mustard
• Rounded 1/4 teaspoon sugar
• Equipment: an adjustable-blade slicer fitted with 1/8-inch julienne blade
• Accompaniment: 4 hamburger buns or kaiser rolls, split and toasted

Instructions

1. Pulse onion and mushrooms in a food processor until finely chopped. Heat 2 tablespoons oil in a 10-inch heavy skillet over medium heat until it shimmers. Add mushroom mixture, 3/4 teaspoon salt, and rounded 1/4 teaspoon pepper and cook, stirring occasionally, until vegetables begin to brown and liquid has evaporated, 8 to 10 minutes. Transfer to a bowl and cool to warm, stirring occasionally, about 15 minutes.
2. Meanwhile, make slaw: Cut celery and apple into 2-inch-long thin julienne with slicer. Whisk together mayonnaise, vinegar, oil, mustard, sugar, rounded 1/8 teaspoon salt, and 1/8 teaspoon pepper in a medium bowl. Add celery and apple, tossing to coat. Let stand 15 minutes to develop flavors.
3. Cut celery and apple into 2-inch-long thin julienne with slicer.
4. Whisk together mayonnaise, vinegar, oil, mustard, sugar, rounded 1/8 teaspoon salt, and 1/8 teaspoon pepper in a medium bowl. Add celery and apple, tossing to coat. Let stand 15 minutes to develop flavors.
5. Finish and cook burgers: Mix buffalo into cooled mushroom mixture with your hands until well combined. Form into 4 (4-inch) patties. Heat remaining 2 teaspoons oil in a 12-inch heavy skillet over medium-high heat until it shimmers. Cook burgers, turning once, about 8 minutes total for medium-rare. Serve burgers, topped with slaw, on buns.
6. Mix buffalo into cooled mushroom mixture with your hands until well combined. Form into 4 (4-inch) patties.
7. Heat remaining 2 teaspoons oil in a 12-inch heavy skillet over medium-high heat until it shimmers. Cook burgers, turning once, about 8 minutes total for medium-rare.
8. Serve burgers, topped with slaw, on buns.