Preheat oven to 350 degrees. Lightly oil an 8- to 10-cup souffle or casserole dish. Cook pasta in a large pot of salted water until barely al dente, about 5 minutes. Drain, and rinse with cold water.
Puree onion, anchovies, artichoke hearts, and capers in a food processor until smooth. Add Parmesan and reserved tuna water, and pulse to combine. With machine running, slowly add 1/2 cup oil, and process until emulsified. Season with pepper. Transfer to a large bowl, and stir in drained tuna, lemon zest, parsley, and pasta. Season with salt and pepper.
Pour pasta mixture into prepared dish. Sprinkle breadcrumbs on top, and drizzle with remaining 2 tablespoons oil. Cover with parchment then foil, and bake until heated through, 40 to 45 minutes. Uncover, and cook until breadcrumbs are lightly browned, 15 to 20 minutes. Let stand for 15 minutes before serving.