In a small saucepan combine stocks and saffron, reserving a few threads for garnish; bring to a boil over high heat. In a large saucepan melt butter over medium heat; while stirring constantly with a wooden spoon add flour and cook 2 minutes. Remove pan from heat and add boiling stock all at once, stirring vigorously with a wire whisk until soup is smooth. Add leeks and celery, return soup to medium-low heat and simmer 20 minutes, or until vegetables are tender.

Meanwhile, slice each scallop into 5 to 6 paper-thin slices. Divide scallops and tomato among 6 wide, shallow soup plates. Using scissors, snip celery leaves evenly on each portion and scatter with reserved saffron threads.

Add sherry to soup and simmer 1 minute more. To serve, ladle about 1/2 cup boiling soup over scallops and vegetables; scallops will cook in hot liquid.