With a mortar and pestle or in a clean coffee grinder, crush the pink peppercorns and fennel seed. Transfer to a small bowl and add garlic, salt, rosemary, thyme and black pepper. Stir to combine. Set aside.
Place the flank steak on a platter or rimmed baking sheet. Rub one teaspoon of olive oil 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 a high heat (about 500 degrees). Place the steak on the grill and turn the heat down to a medium high heat (about 450). Grill 5-6 minutes per side for a medium rare steak.
Remove steak to a clean cutting board, tent with tin foil for 7-8 minutes to rest.
Slice very thinly, on a bias, across the grain.