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First make the relish: heat oil in a large frying pan and add the cumin. Let it cook for a moment, then throw in the bay, garlic, shallots and chilli. Cook for 2 mins until soft but not coloured, then add the celery and tomatoes. Sweat everything down gently for 20-30 mins until the tomatoes lose their shape. Remove from the heat, season and leave to cool. This can be made a few days in advance and kept in the fridge.
To make the burgers, mix the egg yolks in a large bowl with the oyster sauce and soaked breadcrumbs. Season with lots of fresh, black pepper then add the meat, breaking it up a little. Mix everything, trying not to overwork the mixture or the burger will become heavy.
Take the mix and divide in half, then into half and half again so you end up with 8 even pieces. Roll each one into a ball and place a 10 x 10cm square of greaseproof paper on the bottom and top. Put in the fridge for 15 mins to firm.
Heat a griddle pan. Take 4 burger balls from the fridge and push them down so they become discs about the size of a burger bun, about 2cm thick, between the 2 pieces of greaseproof. Peel the paper off one side and slap down in the pan with no oil or seasoning. Peel the top layer of paper off and cook on a very high heat for a couple of mins. In total, a burger this size will take about 5-10 mins to cook, but you need a bit of colour and texture, so exactly how long is up to you. I turn my burgers after 2 mins, but I cook on a very high heat, and I like them a little pink. Keep warm in a low oven while you cook the rest, topping with the cheese slices, if you like, for the final few mins so they melt a little.
Lay the 2 halves of the warm bun in front of you and butter them lightly. Spread the mayo on both sides followed by a little mustard on the top bun and tomato relish on the bottom. Place a slice of tomato on the bottom bun followed by the first burger with the cheese, then some bacon, then the next burger, more relish, lettuce and finally the guacamole. Top with the bun and eat with the gherkins on the side.