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.