Prep: Grate the garlic clove. Grate 1/2 teaspoon fresh ginger. Zest 1/2 lime.
Prepare the marinade by combining the garlic, ginger, lime zest, juice of 1/2 lime, hoisin sauce, reduced-sodium soy sauce, honey, sesame oil, liquid smoke, ground mustard, and Chinese five spice powder in a medium bowl. Add ground cayenne to taste. Stir to combine all ingredients. Set aside 1/4 cup of the marinade for the grilling stage.
Trim excess fat from the flank steak. Place flank steak in a resealable bag and add enough marinade to cover the flank steak. Seal the bag and refrigerate for 24 hours. Flip the bag periodically.
Once the flank steak is done marinating, transfer it to a vacuum-sealed bag or a BPA-free resealable bag. If using a resealable bag, carefully submerge the bag in the water bath until most of the air has been removed, and then seal the bag.
Sous vide. Prepare a sous vide immersion circulator according to instructions and set water temperature for 131°F (55°C).
Sous vide the flank steak for 2 hours. Since there is not much fat to break down, anything longer will not make it more tender, but you can go up to 4 hours if needed.
Chill. Remove the bag from the sous vide water bath and submerge the bag in a large container filled with a 50/50 mix of ice and water for at least 30 minutes to reduce the meat's core temperature. Check to ensure the water stays under 40°F with a digital thermometer, adding more ice if needed. Place the flank steak in the refrigerator until ready to grill. The steak can stay in the refrigerator for up to one week or up to a month in the freezer.
Fire up. Prepare a grill for 2-zone cooking. On a charcoal grill, place a chimney full of pre-heated charcoal briquets on one side of the grill's charcoal grate in order to create direct and indirect cooking zones. Adjust the grill vents to bring the temperature to about 225°F (107.2°C). Add 2 to 3 chunks of your favorite smoking wood to the charcoal for flavor. On a gas grill, adjust the temperature knobs so that one half of the grill is off and the other half is heated enough to maintain a temperature of approximately 225°F (107.2°C) on the indirect side. For a pellet cooker, simple set the temperature to 225°F (107.2°C). Cook. Once the grill is ready, remove the flank steak from the bag, and without patting the meat dry, place it in the indirect zone on the main cooking grate as far from the heat source as possible. Set the lid on the grill with the fully opened top vent positioned directly above the flank steak in order to force the smoke over and around the meat.
Allow the flank steak to smoke until the internal temperature reaches 120°F, approximately 45 minutes.
When the flank steak has reached 120°F, brush both sides with the remaining 1/4 cup of Asian marinade. On a charcoal grill, remove the lid lid and open the vents to get it as hot as possible. For a gas grill or pellet cooker, turn the heat up as much as possible. Sear the steak over high heat, flipping it once to create a nice sear. Once the flank steak reaches an internal temperature of 129°F, remove it from the grill for a perfect medium-rare when sliced and served.
Prep again. Make the hoisin sour cream by combining the sour cream and hoisin sauce in a small bowl and gently blending.
Mince the onion and finely chop the cilantro. Cut the remaining lime into quarters.
Serve. Slice the flank steak against the grain. Build your taco by spreading a little guacamole (optional) on a flour tortilla, and topping with 2-3 slices of flank steak. Serve with hoisin sour cream, as well as the optional chopped onion, cilantro, lime wedges, and pickled jalapeno slices.