Season pork lightly with salt or a seasoning of your choice. Place tenderloin in sous vide bag and remove the air through a vacuum sealer or the displacement method. Drop the bag in the bath for two hours. Remove bag from bath. Take pork tenderloin out of the bag, pat dry with paper towels and season with salt and pepper (or seasoning) to taste.
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.
The displacement method is a technique where you first place your food and marinades in a sous vide bag. Next, submerge the bag in a container of water, and allow the pressure to force all the air out of the bag. Once all the air is out, zip up the bag and you're all set! Either method works for pork tenderloin.
Once the pork tenderloin has been in the water for at least one and a half hours, place a large cast-iron skillet over medium-high heat. Remove the pork tenderloin from the bag. When the skillet is hot, placed the pork tenderloin in the skillet to sear for 2 to 3 minutes per side to create a crust. Remove from the skillet.