Mix the cream and powdered wasabi in a small saucepan. Place over medium heat and bring to a boil. Then set aside.
On a plate spread the black pepper. Press 1 side of the tuna steak onto the black pepper. Repeat with the other steak. Place a cast iron skillet over high heat. Add oil, and when smoking, place the tuna into the pan, pepper side down. Cook for approximately 1 to 2 minutes on each side. As the tuna cooks, the red meat will become whiter. Once you have cooked both sides, the tuna will be almost raw in the middle, thus seared.
Slice the tuna, and arrange on a plate. Drizzle the wasabi sauce around and on top of the tuna. Lightly sprinkle with finely grated ginger. Serve immediately.