Cook barley according to package directions.
Stir in green onion, horseradish, garlic, and 1/4 teaspoon salt.
spread evenly with barley mixture.
Reroll and tie securely.
Place roast, fat side up, on rack in shallow roasting pan; sprinkle with a little salt and pepper.
Insert meat thermometer.
Roast in 325°F oven for 2 1/2 to 3 hours for medium-rare or till thermometer registers 145°F.
Let roast stand about 15 minutes.
Remove strings and carve.