Wisconsin Cauliflower Soup

2 Tbsp. butter 1 medium onion, chopped 1/4 cup flour 1/2 tsp. salt 2 cups milk 3 1/2 cups water 5 chicken bouillon cubes 1 bag frozen cauliflower 1 cup sharp cheddar cheese, shredded 1 cup pepper-jack cheese, shredded * 1 tsp. Dijon mustard In large saucepan, saute onion in butter until golden.  Whisk in flour and salt.  Slowly pour in milk and water. Add bouillon