Trim the lamb cutlets of most of the fat, scraping it away completely from the tips of the bones. Mix together the garlic, thyme, oil and a little salt and pepper in a bowl, then spread over the lamb.
Make the sauce. Put the chillies, garlic, cumin seeds, coriander and oil in a food processor or blender and blend to a thin paste. Stir in the vinegar and season with a little salt.
Cook the baby new potatoes in a saucepan of boiling, salted water for 10 minutes until tender.
Meanwhile, heat a griddle or heavy-based frying pan, add the lamb and fry for about 4 minutes on each side or until cooked but still slightly pink in the centre. Cook for longer if you prefer them cooked through.
Drain the potatoes and tip them back into the pan. Add half the sauce, toss to coat, then transfer the remaining sauce to a small dish.
Transfer the potatoes to 4 warmed plates. Place 2 lamb cutlets on top, and serve the remaining sauce separately for drizzling over the cutlets.