Light your grill and heat for 10 minutes, or preheat your grill pan.
Brush both sides of the leek and the tuna with olive oil; season with salt and pepper. Rub the rosemary on both sides of the tuna. Heat a small sauté pan over medium heat, if using the outdoor grill. If using a grill pan, place the garlic on the grids and toast for about 2 - 3 minutes or until tender. Set aside. (if not using the grill pan, sauté in the sauté pan until tender) When cool enough to handle, remove the garlic from its skin.
Cook the pasta in boiling water according to package directions. Reserve ½ cup of the cooking liquid. While the pasta cooks, begin to grill the leek, turning occasionally until it is tender and slightly charred. Remove and allow to cool slightly. Place the tuna on the grill and grill until cooked to your liking, about 2 - 3 minutes per side for just slightly pink.
Slice the leek into ½ inch slices, discarding any tough pieces. Chop the garlic. Flake the fish into large chunks. Combine in a deep bowl; add the cooked pasta, parsley, capers, lemon juice, lemon zest, and ¼ cup of olive oil; toss well, adding a bit of the pasta cooking water if it seems too dry. Season with salt and pepper; then add the crushed red pepper, if using. Serve.