How to Make It
In a large, heavy pot, bring the wine with the shallots to a boil. Add the mussels, cover and cook over high heat until they open, about 5 minutes. Strain the cooking liquid into a large bowl and reserve. Remove the mussels from their shells, reserving some unshelled for garnish, if desired.
In the large pot, heat the olive oil. Add the chorizo, carrots, leeks and onion; cook over moderate heat, stirring occasionally, until the vegetables are softened, about 8 minutes. Spoon off all but 3 tablespoons of the fat. Add the garlic, diced tomato and saffron and cook over moderate heat until fragrant, about 4 minutes. Add the fish stock and reserved mussel cooking liquid and bring to a boil. Simmer the soup over low heat for 30 minutes.
Add the cream to the soup and simmer over moderately high heat for 3 minutes. Season with salt and pepper. Add the mussels and the thyme and simmer just until the mussels are heated through, about 1 minute. Ladle the soup into bowls, garnish with the mussel shells and serve hot.