Grind peppercorns and cumin seeds (in mortar and pestle, coffee grinder, Magic bullet or use pre-ground if you have too.)
Combine remaining ingredients in blender. Blend well and pour over flank steak in ziploc bag.
Allow to marinate at least 3 hours (preferably overnight).
Pre-heat grill to medium-high heat. Oil the grate to prevent sticking.
Remove meat from marinade and discard marinade and juices. Grill meat for about 5 minutes per side, or until you achieve desired doneness.
Note: if you like your meat a bit on the rare side, try this method. Heat grill to as high as it will go (850 °F (450 °C) ). Throw the meat on for 90-120 seconds. Flip and cook for another 90-120 seconds. Now turn of all the heat and allow to cook for another 4-6 minutes. If your flank has a thicker middle; pound out the meat to a consistent thickness so that everything cooks to the same doneness.
There are 302 calories in 1 serving of Marinated Flank Steak.