Make chipotle chimichurri sauce by placing cilantro, parsley, red pepper flakes, kosher salt, 2 tbsp olive oil, white vinegar, chipotle in adobo, onion, and lime juice in food processor and process until smooth.
Season the steak with salt on both sides. Heat 1 tbsp olive oil over medium-high heat in medium non-stick skillet. Add the flank steak and cook as desired (about 3–4 mins per side for medium rare). Transfer steak to cutting board and slice across the grain.
Warm tortillas for 10–15 seconds on each side on an ungreased pan, then assemble tacos by layering steak and topping with chimichurri sauce.
Garnish with sliced red onions and cilantro, if desired.