With a mortar and pestle or in a clean coffee grinder, crush the Pink Peppercorns (1 tablespoon) and Fennel Seeds (1 teaspoon). Transfer to a small bowl.
Peel and mince the Garlic (3 clove). Prepare the chopped fresh Fresh Rosemary (1 tablespoon) and chopped Fresh Thyme (1 tablespoon).
Add the garlic, Kosher Salt (2 teaspoon), rosemary, thyme and Ground Black Pepper (2 teaspoon) to the bowl. Stir to combine.
Place the Flank Steak (1.5 pound) on a platter or rimmed baking sheet. Rub half of the Olive Oil (2 teaspoon) over one side of the steak.
Sprinkle on half of the peppercorn-herb mixture and press into the steak to adhere. Flip the steak and continue with the remaining olive oil and rub. Press the rub into the steak with your fingers.
Cover lightly with plastic wrap and refrigerate at least 2 hours and up to 8.
Preheat the grill to high heat (about 500 degrees F or 260 degrees C). Place the steak on the grill and turn the heat down to a medium-high heat (about 450 degrees F or 230 degrees C). Grill 5 to 6 minutes per side for a medium-rare steak.
Transfer steak to a clean cutting board, and tent with tin foil for 7 to 8 minutes to rest.
Slice very thinly, on a bias, across the grain.