Heat a large, nonstick skillet over medium-high heat for 5 minutes. Mix together the ground peppercorns, coarse salt, and sugar. Pat the pepper mixture all over the tuna loin to form a crust on all sides. Place the seasoned tuna onto the preheated skillet and sear on all sides, about 5 minutes total. Remove the tuna loin from the heat and cool completely. (Tuna should be nicely seared on the outside and rare on the inside.)
In a small bowl, whisk together the vinegar, soy sauce, honey, sesame oil, peanut oil, and cayenne. Remove the wasabi leaves from the salad blend (the large leaves in the top of the package) and finely chop or cut into thin strips. Place the remaining Asian Wasabi Salad Blend in a large bowl and toss with the wasabi leaves, chives, and parsley. Drizzle the vinaigrette over the leaves, tossing to coat. Season blend with salt and ground white pepper, as needed. Fold in the green onions and orange segments.
Arrange salad on individual plates or a serving platter. Cut tuna into slices and fan out on top of salad. Top with cashews and wonton strips if desired.