Preheat your oven to 200°C (180°C fan) or gas mark 6. Finely chop the rosemary and thyme, and mince the garlic cloves.
Season the lamb rack with salt and black pepper, then sear it in a hot pan with olive oil until browned on all sides, about 5 minutes.
Remove the lamb from the pan. Brush with Dijon mustard, then press the chopped herbs and garlic into the mustard. Coat the lamb with breadcrumbs, pressing gently to adhere.
Place the lamb on a baking tray and roast in the preheated oven for 15-20 minutes for medium-rare, or until your desired doneness. Let it rest for 10 minutes before slicing.
Peel and chop the potatoes into even pieces. Boil in salted water until tender, about 15-20 minutes. Drain well.
Mash the potatoes with butter, then gradually add the cream and milk. Season with salt and pepper until smooth and creamy.
Serve the herb-crusted lamb slices over a generous portion of creamy mash, and garnish with any remaining herbs if desired.
