For the Grilled Watermelon:
In a saucepan, add rice wine vinegar, ginger, lemongrass and sugar, and bring to a boil. Turn it off and let it cool down. Marinate watermelon in this syrup for 2 hours. You must rotate the watermelon every half an hour so all sides get equally marinated. In a hot grill sear each slice of watermelon for 2 minutes on both sides. Remove from grill, take the skin off, cut in large dice and set aside.
For the Dressing:
In a saute pan on low heat, add 1 tablespoon of oil and slowly caramelize the shallots. In a small bowl dissolve the wasabi powder in the water to create a thick paste. Add all the ingredients except the canola oil into a blender and mix until smooth. With the blender running, add the canola oil in a steady stream to create an emulsion. Store in refrigerator up to 3 days.
For the Tuna and assembly:
Season the tuna portions with togarashi, salt and ground pepper. Sear the tuna in canola oil on all sides and cut each into 5 even slices, set aside. Meanwhile, place the mesclun in a mixing bowl and season with salt and pepper. Add daikon sprouts, jicama, wasabi peas and Lemon Wasabi Dressing, and toss it. Slice 16 thin avocado slices. Distribute the salad in equal amounts on 4 serving plates. Top with each 4 avocado slices, and arrange slices of tuna on top. Garnish the plate with the diced watermelon around the salad.