Heat olive oil in a Dutch oven over medium-high heat. Add onions to pan; sauté for 5 minutes or until tender. Stir in sugar, pepper, and 1/4 teaspoon salt. Reduce heat to medium; cook 20 minutes, stirring frequently. Increase heat to medium-high, and sauté for 5 minutes or until onion is golden brown. Stir in wine, and cook for 1 minute. Add broth and thyme; bring to a boil. Cover, reduce heat, and simmer 2 hours.
Place bread in a single layer on a baking sheet; broil 2 minutes or until toasted, turning after 1 minute.
Place 8 ovenproof bowls on a jelly-roll pan. Ladle 1 cup soup into each bowl. Divide bread evenly among bowls; top each serving with 1 cheese slice. Broil 3 minutes or until cheese begins to brown.