Cook the potatoes in a large pan of boiling water for 15-20 minutes until just soft.
Meanwhile, pour the vinegar into a bowl, season and add the herbs, garlic, mustard and oil. Spoon ⅓ of the vinaigrette over the lamb cutlets, reserving the rest for later. Coat thoroughly, cover and leave to marinate while the potatoes are cooking.
When the potatoes are cooked, drain and leave until cool enough to handle, then slice evenly and place in a large dish. Pour over the remaining vinaigrette and leave to infuse for 5 minutes.
Meanwhile, remove excess marinade from the lamb cutlets, and cook for 2-3 minutes on each side under a preheated grill or prepared barbecue. They will be pink at this stage.
Stir the potatoes carefully and serve them warm with the cutlets and a green salad.