Lightly press pepper on top and sides of roast. Place roast in a heavy-duty, zip-top plastic bag or large shallow dish. Combine soy sauce and next 4 ingredients; pour over roast. Seal or cover, and marinate in refrigerator 8 hours, turning occasionally.
Remove roast from marinade, discarding marinade. Place roast, fat side up, on a rack in a shallow roasting pan; insert meat thermometer into thickest part of roast, making sure it does not touch fat.
Bake, uncovered, at 425° for 10 minutes. Reduce temperature to 325°; bake 2 hours and 10 minutes or until thermometer registers 145° (medium-rare) or 160° (medium).
Remove roast to a serving platter; cover and keep warm. Add enough water (about 3/4 cup) to pan drippings to make 1 1/2 cups; return to pan. Combine cornstarch and 1 tablespoon water, stirring until smooth. Stir cornstarch mixture and salt into pan drippings. Cook over medium heat, stirring constantly, until mixture is smooth and slightly thickened. Serve roast with gravy.
