In a large skillet, melt butter; add sliced onions. Sauté until onions are translucent and start to caramelize; add garlic.
Transfer onion mixture to slow cooker.
Mix together flour, Worcestershire, sugar, 1 teaspoon salt and 1/4 teaspoon pepper.
Pour broth, Au Jus mix and Worcestershire mixture into slow cooker, stirring to remove any lumps from the flour.
Cover and cook on low 4-6 hours.
Pour soup into oven-proof bowls. Place on a baking sheet, and put a piece of sliced bread on top of soup. Cover with cheese, and broil in oven 2-3 minutes, until cheese is bubbly.