Step 1

Preheat oven to 400 degrees F. Sprinkle tenderloins evenly with salt and pepper, rub seasoning into meat. If time allows, season tenderloins up to 30 minutes at room temperature ahead of time for most flavor.

Step 2

Heat oil in 12-inch skillet over medium-high heat until just smoking. Place both tenderloins in skillet and sear until well-browned, 3 minutes. Rotate tenderloins 1/4 turn and sear until well-browned, 1-2 minutes. Repeat until all sides are browned. Transfer tenderloins to a foil-lined rimmed baking sheet and roast in oven until tenderloins register an internal temperature of 145 degrees, 10-16 minutes.

Step 3

While pork roasts, add 1 tablespoon butter to the still-hot skillet. Add shallots, water, and sugar. Cook over medium-low heat, stirring frequently, until shallots are browned and caramelized, 7-10 minutes. Set skillet aside off heat.

Step 4

Transfer tenderloins to carving board and tent loosely with foil, let rest 10 minutes.

Step 5

While pork is resting, set skillet back over medium-low heat and add vinegar. Simmer, scraping bottom of skillet with wooden spoon to release browned bits, until mixture is thickened slightly, 5-7 minutes. Add rosemary and any accumulated pork juices, continue to simmer until syrupy and reduced to about 1/3 cup, about 2 minutes longer. Off heat, whisk in mustard and remaining 3 tablespoons butter, 1 piece at a time. Season with salt and pepper to taste.

Step 6

Cut tenderloins into 1/2-inch thick slices and serve with sauce.