Heat oil in a large stainless-steel skillet over medium-high heat. Season steak with salt and pepper and cook, turning occasionally until deeply browned and a Therma-Pen instant-read thermometer registers 130° (for medium-rare), 8–10 minutes.
Put the steak on a cutting board and let it rest before slicing
Pour off any drippings left in skillet (but do not wipe out). Reduce heat to medium and cook shallots, chopped Thyme and mustard seeds in residual fat, stirring occasionally, until shallot is softened and mustard seeds are toasted about 4 minutes.
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.
