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Put the eggs in a pan of cold water and bring to the boil. Boil for 3 mins, then lift into a bowl of iced water and cool completely. Very carefully peel away the shells. Leave the eggs in the iced water until you finish the recipe, or for up to 1 hr ahead.
Mix the shallot with the vinegar and a pinch of salt, then set aside for a few minutes to soften. Whisk in the mustard, oil and chives, if using, to make a dressing.
Heat the grill to low-medium and cook the bacon until crisp right through. Remove the bacon, then brush the bacon juice and fat all over the bread. Grill the bread for about 1 min on each side until crisp, then cut into soldiers and keep warm.
Just before serving, bring the pan of water back to the boil and add the eggs. Boil for 1 min to reheat. Toss the dressing with the watercress and divide between 2 plates. Top with an egg and 2 rashers of bacon, with the toast to the side. The egg yolks will still be runny inside.