This is some Flap Meat I bought at Costco. It was marbled OUT THE WAZOO !! I read up on it (had never cooked it before) and the consensus seemed to be marinate it and cook it no less and no more than medium rare. People said it tended toward toughness.
Out of prudence, I took a Jaccard to it gently. It looked so well-marbled, I could not imagine it being tough. Marinaded in Soy and A1 Bold (my wife didn't like it and we only use it on leftovers anyway.) Got the Grilldome up to about 600F, and gave them about two minutes per side. It was VERY tender, but not as flavorful as I expected for a close kin to flank and skirt. Actually, the NEXT day, I warmed it gently in the convection toaster oven, and it was MUCH more flavorful.. Maybe I was "smoked out" by the hardwood.. That can happen.
Anyway, it was like BUTTER. Don't know what role the Jaccard played nor the marinade (left it about 4 hours) but would def eat this again. It was < 6 bucks per pound.