While the steak is resting, start making the pan sauce by pouring off any drippings left in skillet (but do not wipe out). Reduce heat to medium and cook shallot and mustard seeds in the remnant fat, stirring occasionally, until shallot is softened and mustard seeds are toasted, about 3-5 minutes. Add thyme sprigs, wine, lemon zest, and ½ cup water, and use a wooden spoon to scrape up all the lovely browned bits at the bottom of the pan. Cook, swirling pan occasionally, until liquid is reduced by about half, about 5 minutes. Add butter, swirling pan to melt; season pan sauce with salt and pepper.