Prepare a thermocirculator and water bath according to manufacturer's directions. Set thermocirculator to 135°F (58°C); for medium doneness, set to 145°F (63°C). Allow water bath to come up to temperature.

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

Place tenderloins in a vacuum-sealable bag with 3/4 cup of the mustard mixture. Massage to coat meat evenly. Seal bag with a vacuum sealer.

Transfer vacuum-sealed tenderloins to the water bath. Cover vessel with aluminum foil to reduce evaporation. Cook to desired doneness, 45 to 50 minutes.

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

Heat a large nonstick skillet over medium-high heat. Sear tenderloins until browned on all sides, about 12 minutes total. Transfer to a cutting board to rest for 5 minutes.

Reduce skillet heat to medium. Add remaining 3/4 cup mustard sauce, simmer until slightly thickened, about 1-2 minutes. Stir in cream and remaining 1/2 teaspoon tarragon. Cook until sauce thickens, about 1 minute.