Thaw fish, if frozen. Rinse fish; pat dry with paper towels. Cut fish into 3/4-inch pieces; set aside.
In a medium saucepan cook onion and garlic in hot margarine over medium heat until onion is tender. Stir in water, fish bouillon cubes, lemon juice, bay leaf, dried thyme, fennel seed, and if desired, saffron.
Bring to boiling. Add fish and tomatoes. Return to boiling; reduce heat. Simmer, covered, about 4 minutes or until fish flakes easily with a fork. Discard bay leaf. If desired, garnish with fresh thyme.