Caramelized onion/sweet potato bisque

From my good friend Martha Stookey in Fort Worth, Texas

cup butter
5 large onions, about 16 cups
2 Tbsp brown sugar
4 sweet potatoes (about 2-1/2 lbs) peeled & diced
8 cups reduced-sodium chicken stock
2 T dry sherry
tsp. allspice
tsp. dried thyme
tsp. salt
tsp. white pepper

Melt butter. Add onions & brown sugar. Cook 35-40 minutes, stirring frequently. Caramelize onions to golden brown.
Add all remaining ingredients. Cook 20 minutes until potatoes are tender.
Blend soup until smooth. Return to pot & heat until hot.