Portobello Buffalo Burgers with Celery Apple Slaw

Name: Portobello Buffalo Burgers with Celery Apple Slaw
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