This recipe works with all cuts of steak from ribeye to round, but you don't have to stop at beef. Your pork chops and chicken thighs could use a little buttery pan sauce, too.
Let 1–1½ pounds steak sit at room temperature for as long as possible before cooking (even if it’s only for 10 minutes, that’s fine). Pat steak dry with paper towels.
Heat a medium dry skillet, preferably cast iron, over medium. Season steak while you wait for the skillet to get nice and hot. Sprinkle lots of salt and pepper on both sides of steak—A LOT. Use several generous pinches of salt and several cranks of pepper per side. Press salt and pepper into meat so it adheres.
Pour 1 Tbsp. oil into skillet and swirl to coat. Using tongs, gently lay steak in skillet and cook until underside is deeply browned and a crust has formed, about 5 minutes. Once or twice, lift up steak and reposition over hotter areas of the skillet to encourage even cooking.
Turn steak and cook until reverse side is equally browned and steak is medium-rare (an instant-read thermometer inserted into the thickest part will register 120°), another 5 minutes. If steak has a fat cap or is more than 1" thick, upend with tongs and sear sides until browned. Remove skillet from heat. Transfer steak to a cutting board to rest.
Let skillet cool about 5 minutes. While skillet is cooling, peel and finely chop 1 shallot. Smash 2 garlic cloves with the flat side of your knife and remove peel. Cut ¼ cup (½ stick) butter into 4 Tbsp.-size pieces. Set aside and have at the ready.
Using paper towels, blot around skillet to soak up any blackened oil, but leave browned stuck-on bits behind.
Place skillet back over medium-low heat and add shallot, garlic, 2 sprigs thyme, and 2 Tbsp. butter. Swirl skillet to melt butter and cook, stirring often with a whisk, until shallot is golden brown, 2–3 minutes.
Add ½ cup water to skillet and whisk to dissolve stuck-on bits. Whisk in 2 Tbsp. mustard and 2 tsp. Worcestershire sauce until smooth.
Still working over medium-low heat, add remaining 2 Tbsp. butter and whisk constantly until butter is dissolved and sauce is smooth and thick, about 1 minute. Taste sauce and season with more salt (only if needed) and pepper.
Cut steak against the grain into ½”-thick slices and arrange on a platter.
Spoon pan sauce over steak and serve.