In a medium bowl mix together the vinegar, lemon juice, garlic, shallot, mustard, chili flakes, salt and pepper. Whisk in the olive oil. Add the parsley, cilantro, dill and thyme. Stir. Divide marinade in half and add the capers and pickle to one half. Set that one aside.
Place the flank steak in a dish large enough to hold it and the marinade or put in a large resealable plastic bag. Pour half of the marinade over the steak and flip it around so all sides are covered. Or pour into the bag, seal and swish it around. Cover and refrigerate for at least 3 hours or overnight.
Heat your grill to high. Remove the steak from the marinade and pat dry. Generously salt both sides with Kosher salt and freshly ground black pepper.
Place on the hot grill and cook for approximately 5-6 minutes per side or until a thermometer reaches 130-135°F for medium rare or 120-125°F for rare. Remove from grill, loosely cover with foil and let rest for 10 minutes.
Thinly slice across the grain of the meat. Pour the salsa verde on top or serve on the side.