Place tomatoes and onion in saucepan, and cook for 10 minutes. Press through sieve. Melt butter or margarine in saucepan over low heat. Add flour, salt, and sugar, and stir until smooth. Add sieved tomatoes slowly and, and cook over low heat , stirring constantly, until thickened. Scald milk in top of a double boiler, and add tomato mixture slowly, stirring constantly. Place over boiling water in bottom of double boiler heat for 1 minute, and serve immediately.