loin flap meat
If you aren’t familiar with this cut, it could be because of its name, as it goes by many—flap meat, flap steak, loin tip steak, bottom sirloin, bavette d’aloyau—depending on your butcher and where you live. But whatever it’s called, this boneless cut readily soaks up the flavors of a marinade, has great marbling, cooks quickly, and has a wonderful chewy-tender texture when thinly sliced across the grain. Sirloin tips are a great choice for a quick braise, as they're full of flavor and will have a pleasantly chewy texture after 20 minutes of cooking (further cooking would toughen them).
Some grocers mistakenly label tri-tip steak as sirloin tips. You'll recognize real sirloin tips by the marbling. If the cut looks lean, ask your butcher if it's truly loin flap meat.
Keep refrigerated or freeze for longer storage.