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Make the burgers, pickle and watercress crème fraîche a few hours ahead; chill. Cook and assemble the burgers just before serving. The uncooked burgers can be frozen.
For the red onion pickle, toss the onion with the vinegar, sugar and
a generous pinch each of salt and pepper. Leave for at least 30 minutes.
Preheat the oven to 200°C, fan 180°C, gas 6. Combine the minced beef, shallot, Worcestershire sauce, egg yolk and half the cheddar in a bowl and season with ¼ teaspoon each of sea salt and freshly ground black pepper. Divide the mixture into 8 and shape into patties.
To make the watercress crème fraîche, finely chop the watercress leaves, stir into the crème fraîche with the lemon juice and season.
Of course, skewering the sliders with a cocktail stick or flag isn't essential – but does help hold them together and looks fun.
Whisk the remaining egg with the sugar and a generous pinch of salt until the sugar has dissolved. Brush over the mini rolls and sprinkle over the poppy or sesame seeds. Put the rolls on a baking sheet and bake for 5 minutes until the tops are brown and glossy. Remove from the oven, slice open and keep warm under a clean tea towel.
Preheat the grill to high. Fry the burgers in a little oil in a nonstick frying pan for 5 minutes, turning once, until browned all over and cooked through. Top each burger with a small mound of the remaining cheese, place on a baking tray and grill for 3-4 minutes until melted and bubbling. Serve in the buns, topped with a few slices of the red onion pickle and a spoonful of watercress crème fraîche.