1) For the crust: in a heavy large frying-pan over medium-high heat, add the butter. When the butter is melted, add the shallots and thyme and saute until soft, about 2 minutes. Add the chilli powder and stir. Add the breadcrumbs and saute until golden brown, about five minutes. Season, to taste, with salt and freshly ground black pepper. Remove the crumbs to a small bowl and let cool.
2) For the lamb: preheat the oven to 200°C.
3) In the same large frying-pan over high heat, heat the oil. Working in batches, add the lamb racks to frying-pan, rounded side down. Sear until brown, three to five minutes. Transfer to a clean cutting board, seared side up. Spread two teaspoons of the mustard over each rack. Press the crumb topping into mustard on the racks, dividing equally. Transfer to a rimmed baking sheet. Roast in the oven until desired doneness, 25 minutes for medium well. Let the lamb rest five to seven minutes before slicing. Arrange the sliced lamb on a serving platter and serve.
Notes The ancho chilli is a dried poblano chilli. It is also sold as "pasilla" in the US. It is a deep red colour, and the flavour ranges from mild to pungent. The rich, slightly fruit flavored ancho is the sweetest of the dried chillies. Anchos are often sold whole and can be stemmed, seeded then ground at home in a coffee or spice grinder. They can also be found at most supermarkets, latin specialty markets or online.
Heat 115ml beef stock with a little mint jelly and simmer several minutes, for an easy and tasty sauce to go with your lamb.