For a charcoal grill: Open bottom vent completely. Light large chimney starter filled with charcoal briquettes (6 quarts). When top coals are partially covered with ash, pour evenly over grill. Set cooking grate in place, cover and open lid vent completely. Heat grill until hot, about 5 minutes.
For a gas grill: Turn all burners to high, cover and heat grill until hot, about 15 minutes.
To grill steak: Pat steak dry with paper towels and brush with remaining 3 tablespoons mustard. Clean and oil cooking grate. Place steak on grill and cook (covered if using gas) until well browned and meat registers 120 to 125 degrees (for medium-rare), 4 to 6 minutes per side.
Transfer to platter, tent loosely with aluminum foil and let rest for 5 minutes. Slice steak thin against grain and arrange on platter. Serve with potato and green bean salad.