Venetian Crab Soup

2 tablespoons canola oil
1 small onion, chopped
1 celery rib, chopped
1 small fennel bulb, trimmed and chopped
Shrimp shells from 2 pounds shrimp (reserve shrimp for another use)
1 (28-ounce) can whole tomatoes in juice
1 (1/2-inch) piece ginger, peeled and chopped
1 large pinch saffron threads, crumbled
1/4 teaspoon curry powder
1/2 California bay leaf or 1 Turkish
2 quart chicken stock or reduced-sodium chicken broth
1 (8-ounce) Yukon Gold potato
1 small celery root (celeriac)
1 tablespoon unsalted butter
1 pound jumbo lump crabmeat

Heat oil in a wide 6-quart heavy pot over medium heat until it shimmers. Cook onion, celery, and fennel, stirring occasionally, until golden, 8 to 10 minutes. Add shrimp shells and cook, stirring occasionally, until shells turn pink. Stir in tomatoes with their juice. Simmer, breaking up tomatoes slightly with a wooden spoon, 3 minutes, then add ginger, saffron, curry powder, and bay leaf and simmer 5 minutes. Add chicken stock and boil uncovered 5 minutes. Reduce heat and simmer uncovered 45 minutes.

Discard bay leaf, then purée soup (including shrimp shells) in batches in a blender until finely ground, about 2 minutes per batch (use caution when blending hot liquids). Strain soup through a fine-mesh sieve into a clean pot, discarding solids.

Peel and dice potato. Peel and dice enough celery root to measure 1/2 cup. Add potato and celery root to soup and gently simmer uncovered until tender, about 10 minutes. Remove from heat and stir in butter and crabmeat. Season with salt and pepper.