Combine potatoes, celery, onion, water, seasonings and bouillon in 2 quart casserole. Cover. Microwave on high 7 to 10 minutes. Stir after half the cooking time. Combine 1/4 cup milk with flour until smooth. Stir mixture, milk and butter into potatoes. Microwave on high uncovered for another 7 to 10 minutes, or until thickened. Stirring several times during cooking.