Step 1

Pat the tenderloins dry with paper towels.

Step 2

In a medium bowl, whisk together the brown sugar, mustard, sherry, and rosemary.

Step 3

Place the tenderloins in a baking dish or bowl. Season all over with 1¼ teaspoons salt and pour about one-third of the marinade over top. Turn the tenderloins a few times to coat evenly with the marinade. Marinate for at least 1 hour or as long as overnight (the longer, the better). Cover the remaining marinade and refrigerate until ready to use (it will form the base of the sauce).

Step 4

Preheat the oven to 350°F and set an oven rack in the middle position.

Step 5

Heat the oil in a large cast iron or oven-safe skillet over medium-high heat (see note below if you don't have one of these pans). Add both tenderloins and sear on all sides until nicely browned, 6 to 8 minutes (don't move the tenderloins around between turns; they will brown better if left alone). Transfer the skillet to the oven and cook until the pork registers about 140°F on an instant-read thermometer, 12 to 15 minutes. (To check for doneness without a thermometer, make a slit in the thickest part of the loin; the meat should be mostly white with a slight hint of pale pink, and the juices should run mostly clear.)

Step 6

Remove the skillet from the oven and immediately place an oven mitt or dishtowel over the handle to remind yourself that it is hot (it is easy to forget and burn yourself!). Transfer the tenderloins to a cutting board. Tent with foil and let rest for about 10 minutes, then cut into ¼- to ½-inch slices.

Step 7

Meanwhile, pour the remaining marinade into a small skillet and bring to a gentle boil over medium-high heat. Remove the pan from the heat and, stirring constantly, incorporate the butter one piece at a time. Add ⅛ teaspoon salt and a few grinds of pepper. Taste and adjust seasoning if necessary. Arrange the sliced tenderloin on a platter and drizzle with a bit of the sauce; pass the remaining sauce at the table.