Melt butter in a large saucepan; add onion, celery, green pepper & mushrooms. Cook, stirring frequently, until tender. Stir in potato, chicken broth & thyme. Bring to a boil, reduce heat & simmer, uncovered, 30 minutes or until potato's are tender. Stir in 1 1/2 cups milk, salt & pepper. Combine remaining 1/2 cup milk & flour, stir until smooth. Stir into soup & simmer, uncovered, stirring frequently until slighty thickened.
