Braised flat iron steak with za' atar and sumac

I\'m mixing middle eastern cooking with some American goodies.

2&1/2 lbs. flat iron steak, 11-12 oz. onion, 6-8 garlic cloves, 1/4 c. Za\' Ataar, 1/8 c. sumac, 13-14 oz. fresh tomato, 3-4 oz. green pepper, 1&1/2 ob. peeled potato