1. In a large pan heat the ¼ cup of oil over medium heat. Add the leeks and cook for a few minutes, just until soft. Lower the heat and whisk in the mirin. Cook another minute. Remove from the heat and let the mixture cool slightly. In a mixing bowl, whisk together the leek mixture, Sriracha, honey and orange juice. Drizzle in the remaining oil, whisking until incorporated. Season with salt. Toss enough vinaigrette with the yellow tomatoes, turnips and cucumbers so that they are evenly coated. Let them marinate for 10 minutes to soften.
2. Meanwhile, heat the oil over medium-high heat. Season the tuna steaks on both sides with salt and pepper. Coat each steak with sesame seeds. When the oil is hot, add the tuna and cook for just 2-3 minutes per side (medium rare). Remove the tuna and let rest before slicing each piece against the grain.

To Serve

1. Toss the mixed greens with the rest of the vinaigrette (you may not need to use all of it). Season with salt and pepper to taste. Serve each salad with the marinated vegetables and a piece of sliced tuna steak on top.