Bring a large pot of water to a boil. Add linguine; cook according to package instructions, stirring occasionally, until it is al dente. Remove from heat, and transfer linguine to a colander; let drain, reserving 1/4 cup cooking water.
Make tapenade: In the bowl of a food processor fitted with the metal blade, combine olives, lemon zest, garlic, 2 tablespoons parsley, black pepper, and red-pepper flakes. Process until mixture is finely chopped and combined.
Transfer linguine to a large serving bowl, and toss with reserved cooking water. Add tapenade, tuna, tomatoes, and remaining 1/3 cup chopped parsley; toss well to coat. Serve immediately, garnished with parsley sprigs.