Mix the lime juice and soy sauce in a large bowl and place the flank steak inside, coat well season with salt and pepper and set aside.
In a food processor or blender, add the garlic, parsley, cilantro, lemon juice, and oil with a pinch of salt and red pepper flakes. Pulse until finely chopped.
Preheat a grill or cast iron skillet on the stovetop on medium heat and cook the flank steak to the desired doneness using a thermometer (see below).
Allow flank to rest for 5 minutes before slicing. Slice thin against the grain. Serve the chimichurri sauce over top. Season with salt and pepper to your desired taste.