Let the sirloin sit out at room temperature for 15-30 mins.Pat dry with paper towels and season well with salt and pepper on both sides.Press the salt and pepper into meat so it adheres.
Heat a medium dry skillet (preferably cast iron), and pour inthe tablespoon of olive oil and swirl to coat the bottom of the pan.
Using tongs, lay the steak in the skillet and cook until theunderside is deeply browned a crust has formed, about 5 mins.
Turn the steak and cook the reverse side until it is equallybrowned and steak is medium rare, another 5 mins.
Remove the skillet from the heat, and transfer the steak to acutting board to rest.
Let the skillet cool for about 5 minutes. While it cools peeland finely chop the shallot and garlic. Cut your butter into tablespoon sizedpieces.
Using paper towels, blot around the skillet to soak upblacked oil, but leave the bits stuck in the pan behind.
Place the skillet over med-low heat and add in the shallot,garlic, thyme and 2 tablespoons of the butter. Stir often with a whisk untilshallot is golden brown, about 2-3 minutes.
Add ½ cup of water to the skillet and whisk to dissolve thestuck-on bits. Add in the mustard, Worcestershire sauce and balsamic vinegarand whisk until smooth.
Keep heat on mediumlow and add in the remaining butter. Whisk sauce constantly until the butter isdissolved and sauce is smooth and thick. About 1 min.
Remove the skillet from the heat and season with additional saltand pepper to taste.
Slice the sirloin anddrizzle the pan sauce on top before serving.