- 3 tablespoons packed dark brown sugar
- 2 teaspoons ground cumin
- 2 teaspoons chili powder
- 1 1/2 teaspoons salt
- 1 teaspoon black pepper
- 1 teaspoon dry mustard
- 1 (1 1/4 -pound) flatiron steak (1 inch thick; also called boneless chuck top blade steak)
- 1 medium onion, finely chopped
- 1 tablespoon vegetable oil
- 1/2 cup ketchup
- 2 tablespoons Worcestershire sauce
- 5 tablespoons water
- Stir together brown sugar, cumin, chili powder, salt, pepper, and mustard in a bowl. Pat steak dry and rub all over with 2 tablespoons spice mixture. Reserve remainder for sauce. Marinate steak at room temperature while making sauce.
- Cook onion in oil in a 1- to 1 1/2-quart heavy saucepan over moderately high heat, stirring, until golden brown, 5 to 10 minutes. Stir in ketchup, Worcestershire sauce, and reserved spice mixture and bring to a boil over moderately high heat, stirring occasionally. Reduce heat and cover, then simmer until onions are very tender, about 5 minutes. Transfer to a blender and purée with water (use caution with hot liquids) until very smooth (add additional water, 1 tablespoon at a time, if necessary), then transfer to a bowl.
- Meanwhile, preheat gas grill burners on high, covered, 10 minutes, then reduce heat to moderately high.
- Oil grill rack, then grill steak, covered, 4 to 5 minutes per side for medium-rare. Brush steak all over with some of sauce and grill 30 seconds per side.
- Transfer to a cutting board and let stand 5 minutes. Cut steak into thin slices and serve with remaining sauce.