Combine soy sauce, sherry, garlic, dry mustard, ginger and thyme.
Place roast in large plastic bag; set aside in a deep bowl to steady roast.
Pour in marinade and close bag tightly. Let stand 2 hours at room temperature or overnight in the fridge.
Occaionally press bag against meat in several places to distribute marinade evenly.
Remove from marinade; place roast on a rack in shallow roasting pan.
Roast, uncovered, at 325* for 2-1/2 to 3 hours. Bast occasionally with marinade during last hour of roasting time.
Makes: 10-12 servings.