How to Make It
Heat the oil in a saucepan over medium-high heat. Add the leeks, garlic, and fennel and cook, stirring occasionally, until just softened, 3 to 4 minutes.
Add the sherry, tomatoes and their juices, red pepper, and 1 teaspoon salt and bring to a boil. Reduce heat and simmer for 15 minutes.
Add the fish and mussels and simmer gently until the fish is cooked through and the mussels have opened, 4 to 5 minutes.
Stir in the parsley. Serve with the olives.