Prepare the Burger Patties: In a large mixing bowl, combine the beef mince, breadcrumbs, egg, Worcestershire sauce, ketchup, salt, and pepper. Mix well using your hands until all the ingredients are fully incorporated. Divide the mixture into 4–6 portions and shape them into patties slightly larger than the diameter of your burger buns. Use your thumb to press an indent into one side of each patty to help them maintain their shape during cooking.
Cook the Patties: Heat olive oil in a heavy-based frying pan over medium–high heat. Place the patties in the pan and cook for 10–12 minutes, turning them once halfway through. For an extra flavor boost, add 2 tablespoons of water to the pan in the last 2 minutes to deglaze the sticky bits, coating the patties with the rich flavors. For a smoky, chargrilled effect, cook the patties on a barbecue or a preheated chargrill pan instead. Spray the grill plate with oil just before cooking.
Assemble the Burgers: Toast the brioche buns lightly, if desired. Spread mayonnaise on the bottom half of each bun. Layer with lettuce, tomato slices, red onion rings, and gherkins. Place the cooked burger patty on top and immediately lay a cheese slice over the hot patty to melt slightly. Spread ketchup and mustard on the top bun, then place it on top of the stack.
Serve and Enjoy: Serve the burgers immediately while the patties are hot and juicy. Pair with French fries, coleslaw, or a side salad for a complete meal.