Start by preparing your dressing. In a small bowl, whisk together all of the ingredients for the dressing and let stand for at least ten minutes for the flavors to really combine.
Next, get to work on the salad. The only slightly tricky prep work is segmenting the grapefruit, but I have faith in you. Use a large sharp knife to peel the grapefruits, making sure to remove all the white pith. Next, carefully cut between the membranes of the grapefruit to remove the segments. Relax, the segments don’t need to be perfect. If the Segments are very large, feel free to cut them in half.
Place mesclun in a large salad bowl and add the edamame, carrots, red onion, grapefruit, and avocado. (Always slice the avocado last so that it doesn’t brown.) Set aside while you sear your tuna.
(If you are using 1 large tuna steak, I recommend cutting it into two smaller steaks. It just makes things easier.) Season both sides of the tuna with a generous amount of salt and fresh ground pepper. Heat 1 tablespoon sesame oil in a large pan over medium-high heat. When hot add the tuna steaks and sear for 1 minute on each side. It should have a nice crust but be rare in the middle. (If you like your tuna more cooked, cook it longer. Duh.)
Drizzle the dressing over the salad and gently toss to coat.
Divide the salad among 4 plates. Slice the tuna on an angle and fan it out on top of each salad. Take a bow.