Combine first 6 ingredients in a large saucepan; boil 15 to 20 minutes. Strain tomato mixture through a sieve, mashing vegetables with back of a spoon; discard pulp. Return strained tomato mixture to saucepan, and set aside.
Melt butter in a medium saucepan over low heat; add flour, stirring until mixture is smooth. Cook 1 minute, stirring constantly. Gradually add 1 quart scalded milk; cook over medium heat, stirring constantly, until the mixture is slightly thickened.
Gradually stir milk mixture into reserved tomato mixture. Heat thoroughly. (Do not boil.) Spoon into individual serving bowls. Serve immediately.