Place onions in slow cooker; toss with melted butter and brown sugar.

Cover; cook on low 8 hours to deeply caramelize onions.

Add broth and thyme; cook on high 1 more hour.

Ladle into oven-safe bowls; top each with toasted French bread and a handful of Gruyere.

Broil 2 minutes, until cheese is bubbly and golden.