Prepare the grill for direct cooking over high heat (about 450° to 550°F).
In a small bowl combine the rub ingredients. Lightly brush the tuna steaks on both sides with oil and season evenly with the rub.
Brush the cooking grates clean. Grill the tuna over direct high heat, with the lid open, until cooked to rare doneness, 2 to 3 minutes, turning once. Remove from the grill, let rest at least 10 minutes, and then cut into bite-sized pieces. Meanwhile, bring a large pot of salted water to a boil for the pasta and cook according to package directions. When draining, reserve a few tablespoons of the pasta cooking water.
In a large skillet over medium heat, warm 1 tablespoon oil, add the onion and thyme, and sauté until tender, about 7 minutes, stirring occasionally. Add the garlic and lemon zest and cook for 1 minute. Add the wine, bring to a simmer over medium-high heat, and cook until the skillet is almost dry, 3 to 4 minutes. Add the cream and return to a simmer. Cook until the cream has thickened, about 5 minutes. Remove from the heat, stir in the lemon juice, ¾ teaspoon salt, and ¼ teaspoon pepper, and then fold in the tuna and 2 tablespoons of the chives. Keep warm.
Combine the pasta with the sauce and stir to coat, adding a few tablespoons of pasta water as needed to help the sauce coat everything. Season with salt. Serve right away in shallow bowls, garnished with the remaining 1 tablespoon chives.