Prepare the Chimichurri Sauce: In a medium bowl, combine parsley, cilantro, minced garlic, red pepper flakes, and red wine vinegar. Slowly whisk in the olive oil until well blended. Season with salt and pepper to taste. Set aside to let the flavors meld.
Cook the Steak: Preheat a grill or a grill pan over medium-high heat. Rub the flank steak with olive oil, salt, and pepper. Grill the steak for about 5-7 minutes on each side or until it reaches your desired doneness. Remove from heat and let it rest for 5 minutes.
Slice the Steak: After resting, slice the steak against the grain into thin strips.
Warm the Tortillas: While the steak is resting, warm the corn tortillas on a dry skillet over medium heat for about 30 seconds on each side until pliable.
Assemble the Tacos: Place a few slices of steak on each tortilla. Top with sliced avocado, a sprinkle of red onion, and crumbled feta cheese.
Add Chimichurri: Drizzle a generous amount of chimichurri sauce over the filled tacos.
Serve: Serve the tacos warm with extra chimichurri on the side and enjoy!