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 about 2 minutes. Add the linguine to the skillet and toss. Season with salt and pepper, sprinkle with the parsley and serve.