Step 1

Prepare a thermocirculator and water bath according to manufacturer's directions. Set thermocirculator to 135 degrees F (58 degrees C); if you want pork cooked medium instead of medium-rare, set to 145 degrees F (63 degrees C), and allow water bath to come up to temperature.

Step 2

Combine onion, wine, butter, Dijon mustard, vinegar, lemon juice, whole grain mustard, turbinado sugar, lemon zest, salt, and 1/2 teaspoon tarragon in a medium microwave-safe bowl. Place in microwave and cook on high power until butter melts and sugar dissolves, stirring halfway through, about 1 minute. (This yields 1 1/2 cups sauce).

Step 3

Place tenderloins in a vacuum-sealable bag with 3/4 cup of the mustard mixture and massage contents of bag to ensure meat is coated evenly in sauce, spacing tenderloins in a single layer in the bag. Seal bag with a vacuum sealer according to manufacturer's directions.

Step 4

Transfer vacuum-sealed tenderloins to warm water bath with thermocirculator; cover vessel with aluminum foil to reduce evaporation. Cook to desired doneness, 45 to 50 minutes; meat will be 135 or 145 degrees F (58 to 63 degrees C) when done depending on desired doneness.

Step 5

Remove bag from water bath, transfer meat to a plate and pat dry with a paper towel. Rub with olive oil and sprinkle with pepper.

Step 6

Heat a large nonstick skillet over medium-high heat. Sear tenderloins in skillet until deeply browned on all sides, 3 to 4 minutes on each side. Transfer to a cutting board to rest for 5 minutes. Wipe skillet clean with a paper towel if any drippings are too dark.

Step 7

Reduce heat to medium. Add remaining 3/4 cup mustard sauce to skillet and allow to come to a simmer, reducing until slightly thickened, 1 to 2 minutes. Add cream and remaining 1/2 teaspoon tarragon to skillet, and cook, stirring occasionally, until sauce has thickened enough to coat the back of a spatula, about 1 minute. Slice tenderloins and serve with sauce.