Cut beef and chicken across the grain into thin slices; arrange on serving plate. Thinly slice green end of 1 onion to make 2 tbsp (30 mL); set aside. Slice remaining onions diagonally into 2-inch (5 cm) lengths; scatter over meat and refrigerate. Arrange spinach, watercress, eggplant and mushrooms on serving plates.

In stockpot, bring stock to boil; transfer enough to fill fondue pot three-quarters full. Adjust heat to maintain low boil (cover rest of stock; keep at low simmer to add to fondue pot as needed). Cook beef, chicken and vegetables individually in pot, either in baskets or with fondue forks or chopsticks, then dip in desired sauce.

When meat and vegetables are cooked, strain soup through fine sieve. Measure soup and return to pot; add 1 tbsp (15 mL) sherry per cup (250 mL). Bring to boil; ladle into bowls and sprinkle with reserved onions.