In a large saucepan, combine butter, onions and sugar; saut 20 to 25 minutes or until onions are golden. Stir in flour; continue to cook for 3 to 5 minutes.
Add water and simmer, partially covered, for 30 minutes.
Add salt and pepper, blending well.
Fill 4 oven-proof bowls with soup; top each with a bread slice and sprinkle generously with Parmesan cheese.