Trim the tops of the leeks and half lengthways, leaving the last inch of the root end together. Put the leeks in a frying pan, cover with water and boil for 10 minutes. Drain and cut the ends off the larger leeks. Divide between 4 plates.
Meanwhile, fry the bacon in a pan until golden and crisp. For the dressing, warm the oil and vinegar in a pan over a low heat. Whisk in the mustard, then remove from the heat and stir in half the tarragon. Drizzle the dressing over the leeks, then scatter with the bacon and the remaining tarragon to serve.
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