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Bring a large pan of salted water to the boil and cook the beans for 4-5 mins until cooked, but still vibrant. Drain the beans and tip straight into a bowl of ice-cold water. Drain again and set aside.
Heat a frying pan, tip in the bacon and sizzle for 4-5 mins until it starts to crisp. Throw in the hazelnuts and cook for another few mins until the nuts start to brown. Use a slotted spoon to scoop out the bacon and nuts, but leave behind any juices and residue. Place the pan back on a low heat, slosh in the vinegar and sizzle for a moment, then stir in the cream. Return the beans, bacon and hazelnuts to the pan to warm through and toss them in the dressing. Tip into a bowl and mix in the tarragon just before serving.