"Been searching for that perfect burger recipe, but end up making a meatloaf sandwich? Been looking for that 'Wow!' reaction at your next BBQ? Look no more, this simple recipe does just that. I can't count how many have asked/begged for this recipe at my BBQs, so now it's time to share! Serve on buttered and toasted kaiser rolls and condiments of choice."
Melt butter in a skillet over medium heat and cook and stir onion until nearly translucent, about 4 minutes; remove from heat and cool to room temperature.
Mix the beef chuck, lamb, onion, Worcestershire sauce, salt, and black pepper in a bowl. Divide the mixture into fifths and form each portion into a large burger. Press an indentation in the center of each burger with your thumb to keep burgers flat while cooking. Chill the patties thoroughly, 4 to 8 hours. Allow burgers to warm to room temperature before cooking.
Preheat an outdoor grill for high heat, and lightly oil the grate.
Cook burgers on the hot grill until they show grill marks and the meat is no longer pink inside, about 5 minutes per side. Do not press burgers with a spatula as they cook, and flip only once. Let stand 1 minute before serving.