1. Preheat a grill: If using a charcoal grill, make a fire in your grill. When the charcoals are completely covered in gray ash, hold your hand about 5 inches above the grill grid; count how many seconds you can comfortably leave it there. When you can hold it there for 2 to 3 seconds, you have medium-high heat and are ready to grill. If you are using a gas grill, heat the grill for 15 minutes with the lid closed. Before beginning to cook, reduce the temperature by turning the knobs to medium-high.

2. Place ground sirloin and chuck in a large bowl. Add Worcestershire sauce and dry mustard; season with salt and pepper. Gently shape burgers into six equal patties, each about 3/4 inch thick. Make an imprint in the center of each patty with your thumb. Brush both sides of each patty lightly with olive oil; season with salt.

3. Place burgers directly over heat; cover and cook for 4 minutes. Turn, and continue cooking until desired level of doneness, 4 to 6 minutes more. If using cheese, top each burger with cheese after turning.

4. Meanwhile, butter cut sides of buns and place on grill, cut sides down. Grill until lightly toasted, 1 to 2 minutes.

5. Place burgers on buns and top as desired with onions, lettuce, tomatoes, relish, pickles, ketchup, mayonnaise, and mustard. Serve.