Melt butter in a large saucepan. Gently cook onion and celery in butter for 5 minutes. Stir in flour and cook for 1 minute. Place chopped potatoes in pan. Gradually stir in stock and milk. Bring to a simmer and cook 15 minutes. (or until potatoes are tender) Cut cheese in small pieces. Add cheese and cottage cheese to soup. Stir 2 - 3 minutes or until cheese is melted.
Season with salt and pepper. Sprinkle with chives.