Relish: Place tomato and onion mix in small barbecue cast iron pot, add chutney and garlic. Place pot on barbecue and simmer for 3 minutes. Season with salt and pepper.
Marinade: Combine BBQ sauce and mustard to create a baste for the burgers.
Grill the burgers on the barbecue for 8 minutes, or until cooked through, basting and turning regularly.
Slice the buns in half, lightly brush with olive oil, toast lightly on the barbecue.
Assemble the burger: Layer with relish, sliced tomato and watercress. Add the burgers, gherkins, red onion and more relish.
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