Farfalle Pasta with Pan-Seared Tuna, Lemon, and Garlic

Preheat oven to 400 °F. Pat tuna dry. Rub with 1/2 teaspoon oil and 1/2 teaspoon garlic, then chill, covered, 20 minutes. Heat a well-seasoned cast-iron skillet over high heat until hot, then add 1/2 teaspoon oil, tilting skillet to coat evenly. Season tuna well with salt and pepper, then brown, turning once, about 5 minutes total. Transfer skillet to middle of oven and roast tuna until just cooked through, about 10 minutes. Transfer tuna to a cutting board and flake into bite-size pieces. Keep warm in a bowl, covered. Cook pasta in a 6-quart pot of boiling salted water until al dente. Reserve 1 cup cooking water, then drain pasta in a colander. While pasta is boiling, cook remaining garlic and pepper flakes in remaining 2 teaspoons oil in a small heavy skillet over moderately high heat, stirring, until garlic is pale golden, about 1 minute. Remove from heat. Toss pasta in a large bowl with tuna, 2/3 cup reserved cooking water, zest, juice, garlic and oil, olives, parsley, and salt and pepper to taste (add remaining 1/3 cup cooking water as needed if pasta seems dry). Serve warm or at room temperature.