Place the parsley, onion, garlic, sugar and mustard into the food processor and pulse together until well combined. Add the liver, beef and egg and whizz again to mix. Remove and season well. With wet hands, divide the mixture into 8 patties. Place covered at the bottom of the refrigerator to rest while you cook the onions. Heat a large frying pan with rapeseed oil.
Sauté the onions and sugar on a low light covered for about 20 minutes until soft and golden but not burnt. Pre heat the grill to hot. Lightly coat the burgers with oil. Cook for about 8 - 10 minutes on each side or until cooked through. Toast the ciabatta bread under the grill for about 2 minutes. Spread a little mustard mayonnaise onto the ciabatta roll. Add a generous helping of caramelised onions, some rocket leaves, tomatoes and top with the burger and cheese.
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