1) Whisk together all ingredients, in a small bowl. Cover and refrigerate for at least 30 minutes and up to 24 hours to allow flavours to meld.
For the burgers:
1) Preheat the barbecue to high. Form the meat into 4 (225g) burgers. Season the burgers on both sides with salt and pepper, to taste.
2) Griddle until cooked through, about 3 to 4 minutes on each side. During the last minutes of cooking add 2 slices of cheese to each burger, cover the barbecue and let the cheese soften slightly, approximately 1 minute.
3) Put the burgers on the buns, drizzle with some of the mustard and top with a few sprigs of watercress.