Mix the minced meat with the chopped chilli and garlic. Season with salt and pepper and mix in the beaten egg. Form into 4 burgers and set aside.
Heat the oil in a deep frying pan and slowly fry the onions until they are browned, stirring frequently so they don't burn.
Meanwhile, mix the mayonnaise with the ketchup and toast the burger buns.
When the onions are cooked, remove them from the pan, wipe the pan clean and add a little more oil. Fry the burgers for 3 minutes on each side or until cooked through.
Place some onions on the bottom of the bun, add the burger, cheese, watercress and mayonnaise and serve immediately.