Martha Stewart's Hamburgers

Gently mix beef chuck and sirloin by hand and shape into six 1-inch-thick patties. Cover and refrigerate until cold and firm, about 30 minutes. Heat grill to medium-high. (If you are using a charcoal grill, the coals are ready when you can hold your hand 4 inches above the grill for just 3 to 4 seconds.) When grill is ready, brush grates with oil. Generously season both sides of patties with salt and pepper. Grill, flipping once, until burgers are cooked to desired doneness, about 5 minutes preside for medium-rare. Top each burger with a cheese slice during the final 30 seconds of cooking. Transfer burgers to a platter. Meanwhile, grill buns, cut sides down, until lightly toasted, about 30 seconds. Lightly drizzle onion with oil and toss to coat. Season with salt and pepper. Grill, turning once, until tender, about 2 minutes. Combine mustards in a small bowl, then spread mixture on buns, dividing evenly. Layer bottom halves with lettuce, burgers, tomato, and onion. Sandwich with top halves and serve.