Thaw fish, if frozen; rinse and pat dry with paper towels. Coarsely chop fish. In a food processor bowl or a blender container place half of the fish with 1 tablespoon of the cream; cover and process until smooth, stopping often to scrape the bowl. Remove; set aside. Repeat with remaining fish and 1 tablespoon cream.
In the same bowl or container combine pureed fish, egg whites, parsley, tarragon, and 1/4 teaspoon salt. Cover and process until mixture thickens into a paste. With the processor or blender running, gradually add remaining cream. Process for 30 to 60 seconds or until thick. Cover; chill for 2 hours.
Prepare Tomato Cream; keep warm. Grease a 12-inch skillet. To shape quenelles, dip 2 large spoons into a small bowl of hot water. Using one of the hot, wet spoons, scoop about 2 tablespoons of the fish mixture into an oval mound; use a second hot, wet spoon to smooth the mound into an egg shape. Carefully transfer quenelle to the skillet. Repeat with remaining mixture, making a total of 16. Wet the spoons between the shaping of each quenelle. Do not crowd quenelles in the skillet.
Combine 2 cups water and 1/2 teaspoon salt or use Fish Stock. Add to the skillet. Bring just to simmering. Simmer gently, covered, about 10 minutes or until quenelles are set and light-textured. Remove quenelles from the skillet; drain on paper towels. Serve with Tomato Cream. Top with fresh tarragon, if desired. Makes 8 appetizer servings.
In a 1-1/2-quart saucepan cook green onion in hot butter over medium-high heat for 3 to 4 minutes or until onion is tender but not brown, stirring often. Add tomatoes, sugar, and salt. Cook and stir over medium-high heat for 5 to 8 minutes or until most of the liquid evaporates. Remove from heat. Pour into a food processor bowl or a blender container; cover and process until smooth. Return to the saucepan. Combine milk and all-purpose flour; stir into tomato mixture. Cook and stir over medium heat until thickened and bubbly. Cook and stir for 1 minute more. Makes 1 cup.