How to Make It
Heat oil in a 12-inch high-sided skillet over medium-high. Add leek and fennel; cook, stirring often, until softened, about 7 minutes. Add tomato paste and garlic; cook, stirring constantly, until fragrant, about 1 minute. Stir in clam juice, tomatoes, olives, 1/2 cup water, and 1/4 teaspoon salt; bring to a boil. Reduce heat to medium-low; simmer until slightly reduced, about 10 minutes. Adjust heat as needed to maintain a very gentle simmer.
Sprinkle cod with remaining 1/4 teaspoon salt. Nestle fish into tomato mixture; cover and cook until fish is done, about 10 minutes. Serve cod and tomato mixture over couscous. Garnish with parsley and lemon, if desired.