1. Heat olive oil in a large Dutch oven over medium-high heat. Add onion to pan; sauté 2 minutes, stirring occasionally. Add garlic; sauté 1 minute, stirring constantly. Reduce heat to medium. Add tomato paste to pan; cook 7 minutes or until paste begins to brown, stirring occasionally. Add fennel and next 4 ingredients (through clam juice); bring to a boil. Reduce heat, and simmer 15 minutes or until vegetables are just tender. Stir in shrimp, scallops, and fish. Cover and cook 5 minutes or until fish is done. Gently stir in tarragon, chopped thyme, and pepper. Garnish with thyme sprigs, if desired.