Heat the oil in a saucepan over medium-high heat. Add the onions and cook, stirring, until soft, about 3 minutes. Add the saffron and white wine and stir so the saffron permeates the wine. Add the garlic and cook for 30 seconds. Add the tomatoes, thyme and 1/2 teaspoon of the salt and cook until the tomatoes give up their liquid, about 5 minutes, stirring from time to time. Add tomato paste and cook, stirring, for 1 minute. Add the fish stock, bring to a boil and cook until reduced by half, about 5 minutes. Add the cream and cook until reduced by half, about 3 minutes.
Add the remaining 1/4 teaspoon of salt, the butter and parsley, and whisk to blend. Remove from the heat and discard the thyme sprigs. With a hand-held immersion blender, or in batches in a food processor, puree on high speed.
To serve, ladle the hot sauce onto and around the Crawfish and Salmon Mousses.
Yield: 2 cups