Preheat the grill to high. Meanwhile, season the lamb with salt and pepper. Sprinkle the sugar on a plate. Spread the cutlets with mustard, scatter with rosemary leaves, then roll in the sugar.
Drizzle a little oil over a non-stick baking sheet or tray. Slice the potato as thinly as you can, pat dry on a paper towel, then scatter slices over the baking sheet in a single layer. Drizzle with a little more oil, then grind over some black pepper and scatter with a few more rosemary leaves. Lay the lamb cutlets on top, then grill for 10 minutes, without turning, until the lamb and potatoes are cooked and golden.