Pasta Salad with Tuna, Red Onions, and Frisee

Bring tuna to room temperature. Meanwhile, bring a pot of salted water to a boil. Add pasta, and cook until al dente. Drain, reserving 1 cup water. Drizzle with just enough oil to coat. Spread on a baking sheet, and refrigerate 10 minutes.

Sprinkle tuna with salt, pepper, and 1 tablespoon lemon juice. Heat 2 tablespoons oil in a skillet over medium-high heat. Add tuna, and reduce heat to medium. Cook until cooked through, 5 to 6 minutes per side. Transfer to a bowl, and flake with a fork. Toss with 2 tablespoons oil and remaining lemon juice.

Heat remaining tablespoon oil in the same skillet over medium heat. Add onions, and cook until soft, about 12 minutes, adding reserved pasta water a tablespoon at a time if onions dry out. Add garlic, 1/2 teaspoon salt, and pepper, and cook for 2 minutes. Add vinegar, capers, and red-pepper flakes, and cook until the liquid has been absorbed, about 3 minutes. Add to tuna, and let cool.

Toss together pasta, tuna-onion mixture, frisee, salt, and pepper. Drizzle with oil. Serve at room temperature.