Heat oil in a large Dutch oven over medium-high heat. Add onion, fennel, coriander, garlic, and thyme sprig; sauté 5 minutes. Stir in rind and saffron; add water, clam juice, and tomatoes. Bring to a boil; reduce heat, and simmer for 5 minutes. Stir in salt, fish, and beans; cook 5 minutes. Top with thyme leaves.