Cook the potatoes in boiling, salted water for eight minutes, until soft but still holding their shape. Drain. Char on a hot griddle pan until starting to blacken on one side. Remove and set aside then char the peaches until starting to soften and pick up the grill marks on the flesh.
Blend the parsley, oil, garlic, capers, and mustard until coarse. Check the seasoning and add more if necessary, loosen with 1-2tsp water, then set aside.
Heat a griddle pan until smoking hot, season the lamb well, brown all over then cook for a further few minutes on each side. Leave to rest for five minutes. Slice and serve with the peaches, potatoes and sauce.
250g waxy baby potatoes, halved lengthways
2 peaches, stoned and halved
300g lamb neck fillet (2 x 150g pieces)
4tsp olive oil
1 clove of garlic
2tsp Dijon mustard
Salt and pepper