Depending on the type of blue cheese you prefer, you may not be able to slice the cheese into firm slices. Most blue cheese crumbles and you will have to scatter the crumbled cheese over the burgers. Blue cheese melts nicely and its flavors blend seductively with the beef and the onions.
Combine the beef, salt, and pepper in a bowl. Using your hands, mix well. Form into 6 patties. Refrigerate until ready to grill.
Prepare a charcoal or gas grill. Lightly spray the grill rack with vegetable oil cooking spray. The coals should be hot.
Brush the onion slices with oil and season each with salt and pepper. Grill on the outer edge of the grill for 5 to 6 minutes until the onions begin to soften. Turn and grill for 8 or 9 minutes on the other side of until tender and lightly browned. Lift from the grill, set aside, and keep warm.
Grill the burgers for about 5 minutes. Turn and place a slice of cheese or crumble about 1/2 ounce cheese on top of each burger. Grill the burgers, covered, for 4 to 5 minutes longer, or until the cheese melts and the burgers are medium-well. Top with the onions and serve.