Put melted butter and onions in slow cooker. Stir so that onions are coated with margarine.
Cover and cook on high for 30-35 minutes, until onions begin to slightly cook.
Meanwhile, if using water and bouillion cubes, boil water in microwave and drop cubes into boiling water.
Mix together flour, Worcestershire, sugar, and pepper.
Pour broth, Au Jus Mix and Worcestershire mixture into crockpot. Cover and cook on low for 7-9 hours.
Pour soup into oven-proof bowls (if you have them!) Place on baking sheet, and put a piece of sliced bread on top of soup.
Cover with cheese, and broil in oven for 2-3 minutes, until cheese is bubbly
Cool slightly (your bowls will be hot!), and enjoy..... yummmm!