It's a low-fat cut but you have to marinate it to death or cook it to death for it not to be tough - this makes a lovely spicy shredded beef for tacos, burritos, tostadas, lettuce wraps, etc.
Either use leftovers (I used leftover bbq flank steak) or sear both sides in a pan with a little oil
In the bottom of the crock pot put one sliced onion, one small can of hot mexican tomato sauce (I use 'El Pato') one cup of water, 2 t cumin, 2 t oregano, and 1 T of vinegar (white or balsamic both work great.)
Add seared flank steak. Cover and cook all day on low, remove from the pot and shred the meat with a fork; return to the sauce. My husband loves this on a crusty french roll with avocado; I usually opt for a lighter meal and wrap some in a romaine leaf with a little nonfat sour cream and salsa. Low carb tacos!