Heat the oven to 200°C (390°F). Position a rack in the center of the oven.

Trim any silver skin from the tenderloin using a sharp knife, sliding it under the membrane and pulling it away. Pat the meat completely dry with paper towels.

Season the tenderloin on all sides with kosher salt and black pepper.

In a small bowl, combine the Dijon mustard, whole-grain mustard, honey, apple cider vinegar, minced garlic, and a pinch of salt. Stir until smooth and set aside.

Heat the olive oil in a 10-inch oven-safe cast iron skillet over medium-high heat until the oil just begins to shimmer.

Add the tenderloin and sear on each side for about 2 minutes per side, turning with tongs, until the surface is deep golden brown on all four sides - about 8 minutes total.

Remove the skillet from the heat briefly. Using a pastry brush or spoon, coat the top and sides of the tenderloin generously with the honey mustard glaze.

Transfer the skillet to the preheated oven. Roast for 15-18 minutes, brushing with any remaining glaze halfway through, until an instant-read thermometer inserted at the thickest point reads 145°F (63°C).

The glaze should look deep amber and set rather than shiny and wet. If the surface browns too fast before the internal temp is reached, tent loosely with foil.

Remove the skillet from the oven. Let the tenderloin rest in the pan for 5 minutes before slicing.

Transfer the rested tenderloin to a cutting board. Slice into medallions about 1.5 cm (1/2 inch) thick.

Arrange on a serving plate and spoon any pan juices from the skillet over the slices. Serve immediately.