Heat 2 tablespoons butter in a large heavy pot over medium heat. Add fennel and cook until softened, 5-7 minutes. Stir in garlic and tomato paste and cook, stirring often, until fragrant, about 1 minute. Add white wine and cook until it is reduced by half, about 5 minutes.
Add the mussels, tomatoes and olives, cover, and reduce heat to medium-low. Simmer until mussels open, 8-10 minutes. Remove from the heat and discard any mussels that have not opened.
Stir in remaining 3 tablespoons butter and parsley. Transfer to a large bowl and serve with crusty bread.