1 small bottle (or a glassful, or the equivalent of a cupful) of dry red wine
1 small container (about 1 cup’s worth) beef stock (chicken stock will work too, if you have some lying around)
1 tablespoon whole grain mustard
Season the steak with salt and pepper on both sides (I like very coarsely ground pepper for this) and leave at room temperature.
Heat a heavy-bottomed skillet over low heat for about 5 minutes until hot. Turn the heat to high and leave for 1 minute. Add in a splash of oil (enough to almost coat the bottom of the pan) and leave for 30 seconds. Add the steaks to the pan and cook for 2 1/2 minutes per side (for rare) or 3 minutes per side (for medium rare).
Allow the steaks to rest for 10 minutes before slicing thinly, and against the grain.
To make the sauce
Heat a small saucepan over medium high heat. Melt 1 tablespoon of butter in the pan, add the shallot, and cook until just translucent. Lower the heat to medium, add the wine and cook until almost all the wine has evaporated (it should just be covering the shallots), about 5 minutes. Add in the beef stock and cook until reduced by half, about 5 more minutes.
Remove the pan from the heat and continue once the steak is resting.
Reheat the sauce and add the mustard. Swirl to combine. Season to taste with salt and pepper. Just before serving, add the final tablespoon of cold butter and swirl the pan (don’t stir!) until it’s melted into the sauce.