1 pound albacore or ahi tuna steak, about 3/4 inch thick
Salt and freshly ground pepper
1/2 cup extra-virgin olive oil
1 pound linguine
2 medium red onions, halved lengthwise and thinly sliced crosswise
3 tablespoons finely grated lemon zest
2 tablespoons very finely chopped garlic
1/4 cup fresh lemon juice
12 Calamata olives (2 ounces), pitted and coarsely chopped
2 tablespoons capers, drained and coarsely chopped
2 tablespoons chopped flat-leaf parsley
Season the tuna with salt and pepper. Heat 2 tablespoons of the olive oil in a large skillet. Add the tuna and cook over moderately high heat, turning once, until browned on both sides and pink in the center, about 6 minutes. Transfer to a plate. When cool enough to handle, cut the tuna into 1-inch pieces.
In a large pot of boiling salted water, cook the linguine until al dente; drain.
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.