Add all marinade ingredients to a large Ziploc bag and let meat sit overnight in the refrigerator. Remove meat from refrigerator and bag 30 minutes prior to cooking.
In a medium bowl combine cilantro, garlic, oregano, white wine vinegar, lemon zest and juice, red pepper flakes and kosher salt. Whisk in olive oil.
When ready to cook the meat, start your Traeger grill on smoke with the lid open until the fire is established (4-5 minutes). Set the temperature to 450 degrees F (high) and preheat, lid closed, for 10 to 15 minutes.
Lift the flank steak from the marinade (discard the marinade) and place on the grill grate. Grill with lid closed for 10-12 minutes on each side. However, grilling time will dependent on the thickness and starting temperature of your steak, so it can vary by a lot ~ longer or shorter. Feel free to use the temperature probe and grill to medium-rare.
Transfer the steak to a cutting board and let rest for 5 minutes. Slice thinly on the diagonal across the grain. Serve immediately with warm tortillas and chimichurri sauce.