Crayfish were once so plentiful in Alsatian waters that soups like this one, served at L'Écrevisse in Brumath, were common. Today, much of the crayfish eaten in Alsace is imported from the U.S.
3 3/4 lbs. live crayfish, rinsed
2 tbsp. canola oil
1/2 cup cognac
2 stalks celery, chopped
1 medium carrot, peeled and chopped
6 cloves garlic, peeled and finely chopped
2 tbsp. tomato paste
7 cups fish stock
5 sprigs fresh thyme
3 bay leaves
1 1/2 tsp. arrowroot
1 cup crème fraîche
Salt and freshly ground black pepper
2 tbsp. finely chopped fresh chives
1. Bring a large pot of water to a boil over high heat. Add crayfish, cook for 5 minutes, then drain and run under cold water. Remove meat from tails, reserving shells. Devein crayfish as you would shrimp: Run a paring knife along back of tail and lift out and discard intestine. (This is easiest under running water.) Set meat aside.
2. Heat oil in a large pot over medium-high heat. Add shells and cook, stirring, for 3 minutes. Add cognac, then carefully ignite cognac with a long-handled match. (Keep lid handy so flame can be extinguished if necessary.) Allow alcohol to burn off, about 1 minute. When flame has died down, reduce heat to medium and add celery, carrots, and garlic. Cook, stirring, until vegetables are soft, 10-15 minutes.
3. Reduce heat to medium-low, stir in tomato paste, and cook, stirring, for 2 minutes. Add stock, thyme, bay leaves, and 1 cup water. Simmer for 30 minutes.
4. Strain soup through a fine sieve, discarding shells, herbs, and vegetables. Return soup to pot and simmer over medium heat for 15-20 minutes. Ladle 1/2 cup of soup into a small bowl, stir in arrowroot, mix until dissolved, then whisk thickened mixture into soup. Simmer for 1-2 minutes, reduce heat to medium-low, then whisk in crème fraîche. Season with salt and pepper, stir in crayfish meat, and simmer until heated through, about 2 minutes. Serve warm, garnished with chives.