Preheat a grill on medium-high heat.
Combine the cherries and mustard in a bowl and set aside. Brush the oil on the onion slices and then season the patties and onion with the salt and pepper.
Grill all, flipping once, until the onions are tender and burgers are cooked to your liking, about 10 minutes. Spread the mustard on the heels of the buns. Top with the lettuce, onions, patties, and blue cheese. Top with the crowns of the buns and serve.