Keeping the ground beef cold, gently mix it with ¾ packet onion soup mix and the Worcestershire sauce. Do not overwork.
Divide into two equal patties. Form them slightly wider than the buns and make a broad, shallow indentation in the center of each. Refrigerate until ready to grill.
Prepare the grill for medium-high direct heat with a cooler zone. Preheat, scrape, and lightly oil the cooking grate.
Place the patties over direct heat. Cook without pressing for approximately 4–5 minutes, until browned and ready to release. Flip once and continue cooking, moving to indirect heat if necessary.
Use a thermometer inserted through the side tocheck that the center reaches 160°F (71°C). During the final minute, add twoslices of cheddar to each burger. Toast the buttered buns over gentler heat.
Rest the burgers briefly, then assemble on thebuns with lettuce, tomato, onion, and preferred condiments. Serve immediately.