Portobello Buffalo Burgers with Celery Apple Slaw

INGREDIENTS

• For burgers:
• 1 medium onion, chopped
• 10 ounces portobello mushrooms, trimmed and quartered
• 2 tablespoons plus 2 teaspoons olive oil, divided
• 1 pound ground buffalo
• For slaw:
• 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
• 4 hamburger buns or brioche rolls, split and toasted