minutes Add the butter, thyme, and shallots, reduce the heat to medium, and continue to cook, turning and flipping the lamb occasionally, and basting regularly with the browned butter (to baste, tilt the pan toward you so that the butter pools at the bottom and spoon the butter over the lamb, taking care to hit any underbrowned spots repeatedly), until the lamb is deep golden brown and an instant-read thermometer inserted into the thickest part of each rack section registers 120°F for medium-rare, or 130°F for medium, to minutes longer Transfer the lamb racks to a rack set on a rimmed baking sheet and let rest for 5 minutes To serve, reheat the fat in skillet until smoking and pour over the lamb to warm it Carve and serve immediately