Flank is the rectus abdominis — the abdominal wall — a single flat sheet of long, coarse muscle fiber about an inch thick at the center. It is lean, and the grain runs the full length of the steak in one direction. There are two ways to cook it, hot and fast or long and wet, and how you slice it matters more than either.
It comes off the underside of the animal, behind the plate and below the loin, and its job is holding the abdomen together. That means constant duty: core stabilization plus every breath the animal takes. Muscle that works steadily builds long, thick-bundled fibers with connective tissue woven along them and stores very little fat inside. A whole flank is one muscle, 1.5 to 2.5 lb, roughly 12 inches long and an inch thick in the middle, with the grain visibly running lengthwise. If something sold to you as flank shows two different grain directions, it is not flank.
Flank's chew is a fiber-geometry problem more than a collagen problem — the bundles are long and wide, so a bite taken along them means shearing through several inches of intact fiber. Cutting across the grain at 1/4 inch drops that to a quarter inch of fiber per bite, a larger change in tenderness than anything heat does. It is also lean, roughly 4–6% intramuscular fat, so once collagen contracts at 140°F and wrings water out of the fibers, nothing melts into the gap to cover for it. That is why the two workable strategies sit at opposite ends: reach 130°F fast and get off the heat, or go the other way entirely and hold it above 180°F for 2.5 to 3 hours until the collagen woven along the fibers hydrolyzes into gelatin. The middle — twenty-five minutes at moderate heat — is well past the drying threshold and nowhere near conversion, and that is where most gray, stiff flank steaks come from.
At an inch thick over 600°F-plus coals, the surface browns in about four minutes while the center is still climbing, so crust and a 130°F interior arrive in the same window.
HowSalt 40 minutes to overnight, dry the surface. Direct high heat, 4–5 minutes a side for a 1-inch steak, pull at 125–128°F. Rest 10 minutes, then slice 1/4 inch or thinner across the grain, knife at a 45-degree bias for wider pieces.
Held above 180°F in liquid for hours, the collagen running along the fibers converts to gelatin and the long bundles separate into strands instead of resisting as one sheet.
HowSear it, then 300°F covered with liquid halfway up, 2.5–3 hours, until internal is 200–205°F and two forks pull it apart. Shred with the grain for this one — strands are the point.
Slicing across the grain while the meat is raw solves the fiber-length problem before cooking starts, and 1/8-inch pieces are through in under two minutes, short enough to stay below the drying threshold.
HowFreeze 30 minutes to firm it, slice 1/8 inch against the grain, cook in batches over maximum heat, 60–90 seconds each. A cornstarch and egg-white velvet coating buys another margin.
A long hold at 130–133°F slowly softens connective tissue without ever crossing the temperature where the fibers wring themselves out.
How131°F for 5–8 hours, then dry thoroughly and sear 45–60 seconds a side on the hottest surface available. Still slice it across the grain.
This ruins more flank steaks than any error made on the fire, and it happens after the cooking is over, which is what makes it so galling. The fibers in a flank run the full 12-inch length of the steak in parallel bundles. Cut parallel to them and every piece on the plate is a bundle of intact, inches-long fibers your teeth have to shear lengthwise — the meat can be a flawless 130°F and still eat like rope. Cut perpendicular at 1/4 inch and that same meat comes apart with almost no pressure, because you already did the shearing with the knife. There is no fixing it once it is cut. You cannot un-slice a steak, and re-cutting the strips crosswise just gives you short cubes with the same problem in miniature. Find the grain before the steak goes on the heat, and remember it runs the long way, so you are almost always cutting across the short dimension of the board.
A whole flank runs 1.5 to 2.5 lb and about an inch at the thickest point. Buy it whole rather than in pre-cut strips so you control the slicing. Look for even thickness end to end and a grain you can clearly see running lengthwise. There is a thin sheet of silverskin on one face — have them pull it or take it off with a boning knife, because it is elastin and it will not soften, it just contracts and cups the steak on the grill. Skip anything labeled pre-tenderized or blade-tenderized: the needles have already cut through the fiber bundles, which sounds helpful and in practice means the steak sheds moisture faster and falls apart into shreds. Case-marinated flank is usually older meat with the label doing the work; buy it plain and season it yourself.
Doneness: Fast method: pull at 125–128°F, rest 10 minutes, finish around 130–133°F. Past 140°F it stiffens quickly and there is no fat to disguise it. Braise method: cook to 200–205°F internal and judge by fork — done means the bundles separate under light pressure, not that a probe slides in easily. Either way, slice 1/4 inch or thinner across the grain, except when you are deliberately shredding a braise.
They are different muscles. Flank is the abdominal wall: about an inch thick, one clean grain direction, lean, fairly mild. Skirt is the diaphragm: half that thickness, much coarser and more open grained, with noticeably more intramuscular fat and a stronger mineral flavor. Skirt takes marinade further because of the open grain and cooks in 2–3 minutes a side. Flank is thicker, easier to slice into neat pieces, and better for stir-fry or a fanned steak on a platter. For fajitas and carne asada, skirt.
Almost always the slicing. The fibers run the length of the steak, so cutting along them leaves inch-long intact bundles in every bite — cut across at 1/4 inch instead. Second cause is temperature: at 4–6% fat, anything past 140°F stiffens and nothing is melting to soften it. Third is landing in the dead zone, 20 to 30 minutes at moderate heat, which is far too long for a steak and far too short to convert any connective tissue.
Barely, and not the way it is usually described. Acidic marinades — citrus, vinegar, wine — travel about 1/8 inch in several hours and only denature protein at the surface, so the interior is untouched. What they mostly do is flavor the outer layer. Salt penetrates further and does more for moisture retention, which makes a dry brine from 40 minutes to overnight a better use of the same time. Enzymatic marinades with pineapple, papaya, or fresh ginger really do break down protein, but they work outside-in and turn the surface mushy past about four hours.
London broil is a method, not a cut: marinate, broil or grill hot, slice thin across the grain. It was originally done with flank. Most packages labeled London broil in a US supermarket today are top round, which is leaner, thicker, and tougher than flank and needs the slicing discipline even more. Read the fine print for the actual cut name. If it says top round, treat 130°F as the ceiling and slice it as thin as your knife will go.