Finely dice the shallots. Heat a heavy aluminium skillet or a cast iron skillet over a medium flame. Remove the moisture from steak with paper towels. It's also recommended to score the side which contains the fat in 2 places, so as to prevent the steak from curling up. Smear a small amount of butter on one side of the steak. Place this side down in the hot pan and sear until brown. Turn the steak, and season the browned side with salt and pepper. Sear remaining side until brown. With a pair of tongs, sear and render the fat on the fat side of the steak. Test for doneness by touching. The steak should not be too firm or too elastic for medium rare. Season remaining side of steak. Move it to a plate.
Add the butter to the pan. Place the shallots into the pan, sweating them down over a medium-low flame. Do not burn the shallots! Deglaze the pan with wine. Reduce wine to a taut consistency. Add the heavy cream and reduce. Pour the collected blood and juices from under the steak into the pan. Stir in mustard, season with salt and pepper, and reduce to desired consistency. Do not cook mustard for more than 1 minute, as it may become bitter. Ladle the pan sauce over the waiting steak.