Mix halibut, lemon juice and 2 ts oil in small bowl. Let mixture stand while preparing soup. Heat 2 tb oil in heavy large saucepan over medium heat. Add onion and cook until translucent, stirring occasionally, about 10 minutes. Squeeze tomatoes through fingers to break up while adding to saucepan.
Add clam juice, wine, bay leaf, orange peel, fennel and pepper flakes. Simmer 10 minutes. Season with salt and pepper. Add orzo and cook 5 minutes. Add clams to soup.
Cover and cook 5 minutes. Add fish. Cover and cook until fish is opaque and clams open, about 4 minutes longer. Discard bay leaf and orange peel. Spoon soup into bowls. Garnish with parsley and serve.