Chicken with sumac

2 tablespoons olive oil 1 1/2 kg chicken thigh fillets , skinned and all visible fat removed (breast fillets could also be used, but don't over cook) 5 large[...]

8 boneless skinless chicken thighs , cut into 2-inch chunks 8 medium red potatoes, cut into quarters 1 small onion , peeled and cut into eigths 1/2 teaspoon[...]