1. Have beef at room temperature. In a medium-size bowl, lightly knead the sirloin with the chuck and loosely form into four patties about ¾-inch thick. Very generously season with salt and pepper. Place on a plate that has been lined with plastic wrap.
2. When the fire in the grill is medium hot, oil the grill’s grate. Grill the burgers for about 10 minutes, turning once, for medium. Move away from the heat.
3. Brush the cut sides of the buns with melted butter and grill for about a minute.

Beer Cheese Sauce:

1. Toss the cheese with cornstarch in a small bowl.
2. Rub the garlic around inside of medium saucepan; drop the garlic into the pan and add beer. Bring to a boil.
3. Add the cheese-cornstarch mixture in batches and stir until melted. Bring to a boil again.
4. Add mustard and simmer until thickened. Keep warm over low heat.
5. Ladle cheese sauce over cooked burgers.