In a small bowl, combine the mayonnaise, ketchup, chilli sauce, and mustard. Stir thoroughly until the mixture is smooth and well blended. Set aside.

Heat a small amount of butter in a pan over medium heat. Place the burger buns cut-side down and toast them until golden brown and slightly crisp. Remove from the pan and set aside.

Preheat a large pan or griddle over high heat until it is very hot.

Divide the minced beef into balls of approximately 75g each. Place the balls on the hot surface and, using a spatula or burger press, flatten them into thin patties. Season each patty with salt and pepper, then scatter the julienned onions on top.

Cook the patties for 2–3 minutes without moving them, until the edges turn white and the underside develops a light crust.

Carefully flip the patties over. Immediately place a slice of cheese on each one and cook for a further 2–3 minutes, until the cheese melts and the patties are cooked through but still juicy. Avoid overcooking to prevent dryness.

Assemble the burgers by spreading a generous layer of the prepared burger sauce on the bottom half of each bun. Add lettuce if desired, then place the cooked patty on top. Top with slices of tomato and jalapeños as preferred. Cover with the top bun.

Serve immediately while hot, allowing the melted cheese and fresh toppings to shine in every bite.