This is the first flat iron I've cooked. I've had them before in restaurants and they are quickly becoming a new favorite.
I started with a 13oz flat iron
I added a rub with some coffee, a bunch of different ground chiles, salt/pepper, onion/garlic, and a little sugar. Then I put it in a bag and poured a little kefir (a thick, yogurt type drink) over the top and squished it around to marinate for about 4 hours. Kefir is a little acidic and a natural tenderizer, kind of like yogurt marinades for tandoori chicken.
I missed the grill shots, everything got done quickly. The steak took just over 3 minutes per side. We had it with grilled green onions, yellow bell peppers, and some charred and roasted hatch chiles that were all marinated in olive oil and lemon juice.
Generally the yogurt dries up and helps make a great crust, but it was a little cooler tonight and I would have needed more fuel for a good char. Once it was good and medium rare I pulled it rather than overcook it chasing the char.
Thanks for looking!