Meanwhile, heat the remaining 1/4 cup plus 2 tablespoons of olive oil in a large, deep skillet. Add the onions, cover and cook over moderate heat, stirring occasionally, until softened, about 8 minutes. Add the lemon zest and garlic and cook over moderately high heat, stirring, until fragrant, about 3 minutes. Add the lemon juice, olives, capers and tuna and stir until heated through, about 2 minutes. Add the linguine to the skillet and toss. Season with salt and pepper, sprinkle with the parsley and serve.