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Mix the lamb, grated onion and herbs in a bowl with some seasoning, then divide the mix into 4 and shape into burgers.
Heat a griddle pan until hot, rub the burgers with oil, and cook for 5-6 mins on each side, or until cooked through.
Meanwhile, mix the mayonnaise and beetroot with some seasoning. Fill the bread rolls with some watercress, a burger, a dollop of beetroot mayo and a few onion slices. Serve immediately.
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