1. 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.

2. 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.

3. 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.

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