For the crust:
In a heavy large skillet 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 chile powder and stir. Add the bread crumbs and saute until golden brown, about 5 minutes. Season, to taste, with salt and freshly ground black pepper. Remove the crumbs to a small bowl and let cool.
For the lamb:
Preheat the oven to 400 degrees F.
In the same large skillet over high heat, heat the oil. Working in batches, add the lamb racks to skillet, rounded side down. Sear until brown, 3 to 5 minutes. Transfer to a clean cutting board, seared side up. Spread 2 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 5 to 7 minutes before slicing. Arrange the sliced lamb on a serving platter and serve.
The ancho chile is a dried poblano chile. It is also sold as "pasilla" in the US. It is a deep red color, and the flavor ranges from mild to pungent. The rich, slightly fruit flavored ancho is the sweetest of the dried chiles. 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 1/2 cup beef broth (stock) with a little mint jelly and simmer several minutes, for an easy and tasty sauce to go with your lamb.