Heat 1 teaspoon oil in a large saucepan over medium heat. Add onion and next 6 ingredients (onion through garlic); cook 7 minutes or until tender, stirring frequently. Stir in water, tomato paste, and clam juice; bring to a boil. Reduce heat, and simmer 45 minutes.
Strain mixture through a sieve over a bowl; discard solids. Return mixture to pan; bring to a boil. Add potato; cook 5 minutes. Add fennel; cook 4 minutes. Remove from heat, and keep warm.
Heat 1 teaspoon oil in a large nonstick skillet over medium-high heat. Sprinkle fish with salt and pepper. Add fish to pan; cook 3 minutes on each side or until fish flakes easily when tested with a fork.