Step 1In a large saucepan, combine onions, water, celery, bouillon, salt and pepper. Bring to a boil. Reduce heat; cover and simmer for 10 minutes or until vegetables are tender. Transfer mixture to a blender; puree until smooth. Return to saucepan.
Step 2Combine the flour and milk until smooth; gradually stir into onion mixture. Bring to a boil; cook and stir for 2 minutes or until thickened. Reduce heat; stir in cheese until melted. If desired, garnish servings with celery leaves, croutons, and extra cheese.
