Once your tenderloin comes out of its sous vide bag, it is technically completely cooked, but it will not be very appetizing. Without some degree of high-heat cooking, it won't have any of the wonderful brown color and flavor that come with traditionally roasted meats. You need to add that sear after cooking.
When pork is mostly browned, add butter and reserved half of garlic, shallots, and herbs (if using) and cook, tilting pan and using a spoon to baste pork with the flavorful butter, until pork is well browned on all sides, about 30 seconds longer. Transfer pork to a rack set in a rimmed baking sheet and pour the drippings on top.
Flavor-wise, it's easy to add aromatics or spices to the sous vide bag along with the pork, building that flavor right into the meat. (You can reinforce the flavor with more aromatics when you subsequently sear the roast.) As for texture, with sous vide cooking, overcooked meat is a thing of the past.
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.