Preheat the oven to 400 degrees F. Combine the first four ingredients on a plate. Cut the lamb into 2, 4-rib racks. Brush the top and sides of the lamb with the mustard.
Dip the mustard-coated portions of the lamb in the bread crumb mixture, pressing it on to help it adhere. Set the lamb in a small roasting pan. Roast for 22 to 25 minutes for rare, 25 to 28 minutes for medium rare, and 30 minutes for medium. Once cooked, set the lamb on a plate, tent with foil and allow to rest 5 minutes.
Place the roasting pan on the stovetop over medium-high heat. Add the wine and reduce by half. Whisk the flour and stock together in a bowl; add to the pan. Simmer until a thickened sauce forms, about 2 minutes. Serve the lamb racks whole, or slice and fan on dinner plates. Serve the sauce alongside or pooled under the lamb.