My absolute favorite way to marinate flank steak is with a sweet and spicy Thai-style sauce. I make mine with palm sugar (brown sugar will do fine), dried Thai chili flakes, fish sauce, garlic, and lime juice, which I then split in half, reserving half to toss with the meat after cooking, and adding some oil to the other half to use as a marinade. Once the meat is cooked, rested and sliced, I toss it all together with some reserved marinade, and a big herb salad with shallots and bean sprouts. It gets an awesome caramelized, charred crust on the grill from all the sugar, and the great thing is that it’s delicious even when cold, making it the perfect dish for potlucks or relaxed backyard parties.
Note: An equivalent weight of skirt, flap, or hanger steak can be used in place of flank steak.