Thaw fish, if frozen. Rinse fish; pat dry with paper towels. Cut fish into 1-inch chunks.
In a 5- to 6-quart Dutch oven melt butter over medium heat. Add chopped fennel; cook 5 to 6 minutes or until soft, stirring occasionally. Add garlic; cook and stir 1 minute.
Add broth, potatoes, salt, and pepper. Bring to boiling; reduce heat. Simmer about 10 minutes or until potatoes are tender.
Stir together 1 cup of the half-and-half and the flour; add to soup. Cook and stir until slightly thickened and bubbly. Stir in salmon. Return to boiling; reduce heat. Simmer gently 3 to 5 minutes or until salmon flakes easily when tested with a fork. Stir in remaining 1/2 cup half-and-half; heat through. Stir in lemon zest. Coarsely chop fennel fronds; top servings with fennel fronds.