1-1/2 pounds ground chicken (preferably thigh meat)1/4 cup mayonnaise4 teaspoons Dijon mustard1 large sweet yellow onion, cut crosswise into 1/4-inch slicesExtra-virgin olive oilKosher saltGround black pepper4 slices blue cheese, each about 1 ounce8 slices rye bread1. Shape the ground chicken into four equal-sized patties, each about 3/4 inch thick. With your thumb or the back of a spoon, make a shallow indentation about 1 inch wide in the center of each patty so the centers are about 1/2 inch t...