How to Make It
Place barley in a spice grinder, and pulse until finely ground. Heat a nonstick saucepan over medium-high; add barley, and cook, stirring constantly, until toasted and very fragrant, about 5 minutes.
Reduce heat to low; add mirin and sake, and cook, stirring to loosen browned bits from bottom of pan. Add soy sauce, brown sugar, sherry vinegar, garlic, scallions, and ginger, and cook until mixture is thickened and slightly reduced, about 15 minutes. Pour mixture through a wire-mesh strainer into a bowl, discarding solids. Set sauce aside.
Sprinkle roast with salt. Place on a plate, and let stand 40 minutes at room temperature.
Open bottom and top vent of a charcoal grill completely. Light a charcoal chimney starter filled with briquettes. When briquettes are covered with gray ash, pour them onto bottom grate of grill, and then push to 1 side. Sprinkle roast with pepper, and place on oiled grate over unlit side. Cover and grill, with all vents open, flipping roast and checking internal temperature every few minutes, until a thermometer inserted into thickest portion registers 105°F, 20 to 25 minutes.
Transfer roast to lit side of grill over coals; brush with some of the sauce, and grill, flipping every 30 seconds to 1 minute, brushing each time with sauce, until a deep char has developed and internal temperature registers 125°F, about 10 minutes (for medium-rare) or to desired degree on doneness.
Transfer roast to a cutting board, and let rest at least 5 minutes. Carve and serve immediately with remaining sauce.