Remove the beef tenderloin from the refrigerator 30–45 minutes before cooking so it comes to near room temperature. Pat dry thoroughly with paper towels.

Season

Evenly rub kosher salt, black pepper, and garlic powder over all sides of the tenderloin. Press the seasoning gently so it adheres.

Sear

Heat a heavy skillet (cast-iron preferred) over medium-high heat until very hot. Add 1 tablespoon olive oil and 1 tablespoon butter. When the fat is shimmering and the butter foams, place the tenderloin in the pan. Sear without moving for 2–3 minutes until a deep brown crust forms. Turn and sear the other sides, about 2 minutes per side, including the ends, so total searing time is about 8–10 minutes.

Finish in oven

Preheat oven to 400°F (200°C). After searing, transfer the skillet (or place the tenderloin on a rimmed baking sheet if your pan is not oven-safe) to the oven and roast until the internal temperature reaches 125°F (51°C) for medium-rare, about 8–12 minutes depending on thickness. Use an instant-read thermometer inserted into the center to check doneness.

Rest

Remove the tenderloin to a cutting board and tent loosely with foil. Rest 10–12 minutes (carryover cooking will raise temperature ~5–8°F). This keeps juices in the meat and makes slicing easier.

Slice

Using a sharp carving knife, slice the rested tenderloin into 1/2-inch thick medallions. Arrange on a warm serving platter.

Pan Sauce

Prepare aromatics

While the beef rests, place the skillet used to sear the tenderloin back on medium heat. Add the remaining 1 tablespoon butter. When melted, add the minced shallot and sauté 1½–2 minutes until softened and translucent. Add the minced garlic and cook 20–30 seconds until fragrant, taking care not to burn.

Deglaze

Pour in the red wine (or 1/4 cup beef stock if not using wine) and scrape the bottom of the pan with a wooden spoon to loosen browned bits (fond). Let the wine reduce by about half, approximately 1–2 minutes.

Build sauce

Add the 1/2 cup beef stock, Dijon mustard, and thyme. Stir and bring just to a simmer. Let the sauce reduce slightly for 2–3 minutes until it thickens enough to coat the back of a spoon.

Finish and season

Whisk in the last tablespoon of butter off heat to enrich and slightly glossy the sauce. Taste and add 1/4 teaspoon salt and 1/4 teaspoon freshly ground black pepper, or adjust to taste. Stir in chopped parsley.

Serve

Spoon the warm sauce over the sliced beef medallions or serve on the side. Garnish with additional thyme sprigs if desired.