Cook potatoes, celery and onion in a small quantity of boiling salted water until tender; do not drain. Cut sausage in small pieces and fry slowly; pour off most of the fat leaving 2-3 tablespoons. Add flour to sausage and mix well. Add milk all at once and cook, stirring constantly until thickened. Add potatoes. Season with salt and pepper. Reheat slowly. If held for a period of time, keep hot in a double boiler. Serves 6.