Heat oil in a large Dutch oven over medium-high heat. Add onion to pan; sauté 3 minutes. Add fennel; sauté 3 minutes. Add garlic; sauté 5 minutes or until vegetables are tender.
Add broth, tomatoes, and thyme to pan; bring to a boil. Partially cover, reduce heat, and simmer 35 minutes. Remove pan from heat; cool 5 minutes. Discard thyme sprig. Place half of tomato mixture in a blender. Remove center piece of blender lid (to allow steam to escape); secure blender lid on blender. Place a clean towel over opening in blender lid (to avoid splatters). Blend until almost smooth. Pour into a large bowl. Repeat procedure with remaining tomato mixture. Stir in salt and pepper; keep warm.
Place bread slices on a baking sheet. Sprinkle 1 tablespoon cheese on each. Broil 2 minutes or until cheese melts.