Remove the flank steak from the refrigerator 30 minutes prior to cooking to bring it to room temperature.
In a medium bowl, add the scallions, chives, olive oil, basil, parsley, lemon zest, lemon juice, lime zest, lime juice, mustard, red pepper flakes and 1/2 teaspoon kosher salt. Stir to combine and set aside.
Heat a grill pan over medium-high heat. Drizzle a small amount of olive oil onto one side of the steak and use a silicone brush to spread it all over the steak. Sprinkle with a nice pinch of kosher salt and pepper. Transfer the meat to the grill pan, seasoning side down. Drizzle the other side of the steak with a small amount of olive oil and once again brush it to cover and sprinkle with salt and pepper. For medium-rare, cook the steak until the internal temperature is 135 degrees F, 4 to 5 minutes per side. Remove the steak to a plate and let it rest for 10 minutes before slicing.
Slice the steak against the grain. Add the slices to a platter and spoon on the lemon-lime herb sauce. Serve extra sauce on the side.
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