Cut the tuna into 2 inch logs, 1 1/2 inches wide and 1 1/2 inches thick. Season the tuna with salt and pepper. In a small bowl, whisk the wasabi and water together to form a loose paste. Using the potato threader, cut the potato into thin curl-like pieces. Season the potatoes with salt and pepper. Divide the potato curls between the tuna logs in individual piles. Brush each piece of tuna with the wasabi mixture, coating each side of the tuna completely. Place a piece of the tuna in the center of each pile of potato curls. Wrap the potatoes entirely over each piece of tuna, tightly. In a large saute pan, heat the olive oil. When the oil is hot, pan-fry the tuna until golden brown, about 2 to 3 minutes on each side. Fry the tuna in batches. Serve the tuna on a platter with a drizzle of sesame oil, chervil and a dollop of caviar.