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Heat oven to 200C/180C fan/gas 6. Rub the pork with a tbsp of oil, add some seasoning, then seal the outside of the meat all over in a hot frying pan. Transfer the pork to a baking sheet, brush the outside with the mustard, then press on half of the tarragon and cook for 20 mins. Remove from the oven and cover with foil for 5 mins.
Meanwhile, cook the lentils in the stock until just tender, about 12 mins, drain, then toss with the remaining olive oil, half the lemon juice and the apple.
To make the aioli, mix the mayo, garlic, lemon zest and remaining juice and tarragon with some seasoning, then set aside. Slice the pork into thick slices and serve on a platter with the lentils, a bowl of aioli to dip into and some green beans.