In a small bowl, whisk together the marinade ingredients
Place flank steak in a large airtight container or gallon-size ziplock bag. Pour the marinade ingredients over the steak, coating it as evenly and fully as possible.
Close container or bag and place in refrigerator. Leave to marinate overnight, or for at least 8 hours.
Fire up your smoker to 225°F (107°C). If you are using a charcoal grill, ensure you set up for 2-zone indirect cooking.
Remove flank steak from marinade and use paper towels to pat dry any excess sauce
Place flank steaks on smoker grates. Close smoker lid and smoke for 90 minutes, or until internal temperature reaches 135°F (57°C).
Remove flank steak from smoker and place on chopping board. Tent in foil and leave to rest for 5-10 minutes.
To serve, thinly slice the flank steak against the grain at a 45-degree angle.
Serve with your choice of chimichurri, sour cream, grilled asparagus, or mashed potato.