Pat the beef tenderloin medallions dry with paper towels. Season both sides evenly with kosher salt, 1/2 tsp freshly ground black pepper, and garlic powder. Let sit at room temperature for 20–30 minutes to relax the meat and promote even cooking.

Preheat pan

Heat a heavy skillet or cast-iron pan over medium-high heat until very hot, about 3–4 minutes. Add 1 tablespoon olive oil and swirl to coat.

Sear medallions

Place medallions in the hot pan without crowding (work in batches if needed). Sear undisturbed for 2–3 minutes until a deep brown crust forms. Flip and add 1 tablespoon unsalted butter, crushed garlic cloves, and thyme sprigs to the pan.

Baste and finish

Tilt the pan slightly and use a spoon to baste the tops of the medallions with the melted butter for another 2–3 minutes for medium-rare (internal temperature ~125–130°F / 52–54°C). Adjust time by 1–2 minutes for desired doneness. Transfer seared medallions to a warm plate and tent lightly with foil to rest for 6–8 minutes. Repeat for remaining medallions, adding remaining 2 tablespoons butter to the pan as needed.

Truffle Glaze & Finish

Make pan glaze

Lower heat to medium. Discard thyme stems and garlic solids if overly browned but leave fond in the pan. Pour 1/4 cup beef demi-glace into the pan to deglaze, scraping up browned bits with a wooden spoon. Add 2 tablespoons black truffle butter and stir until melted and integrated.

Season and enrich

Whisk in 1 teaspoon black truffle oil and 1 teaspoon balsamic vinegar. Simmer gently for 1–2 minutes until slightly thickened to a glossy glaze. Taste and season with 1/4 teaspoon sea salt and 1/4 teaspoon freshly ground black pepper, adjusting as needed.

Coat medallions

Return rested medallions to the pan for 30–45 seconds per side, spooning the truffle glaze over them to warm and coat evenly. Remove medallions to a serving platter.

Finish and garnish

Spoon any remaining glaze over the medallions. Sprinkle chopped fresh parsley and, if using, finely shaved black truffle or truffle zest over the top. Add a final light drizzle (about 1/4 teaspoon total) of black truffle oil over the plated medallions for extra aroma.

Accompaniment (optional)

Arrange mini Yorkshire puddings or toasted baguette rounds on the serving platter. Place medallions atop or beside these rounds and scatter microgreens or watercress around the plate. Serve immediately with lemon wedges on the side for those who want a touch of brightness.