In a large salad bowl, whisk together lime juice, fish sauce, sugar, and sesame oil. Taste and adjust seasonings with salt and pepper. Cut cabbage in 1/2 lengthwise and slice into thin ribbons. Place cabbage, carrots, basil, mint, and cilantro in the bowl with the dressing. Toss to combine. Set aside.
Pour the sesame seeds out onto a plate. Press the tuna into the seeds on both sides, pressing gently so the sesame seeds stick.
Heat the olive oil in a 12-inch non-stick pan over medium high heat. Add fish to pan and cook until rare, about 2 minutes per side. Alternatively, cook until done to your likeness. Slice thin and distribute among 4 serving plates. Serve with a generous portion of Napa cabbage slaw.