In a spice grinder or with a mortar & pestle, grind peppercorns, sesame seeds, and salt together until it becomes a relatively fine powder. Spread mixture on a plate.
One at a time, press tuna steaks into the spice mixture, coating both sides. Use hands to press additional spice mixture into any areas missed, Set tuna steaks aside.
In a saucepan (NOT over heat!), whisk together wasabi powder and water until smooth, allow to sit for 5 minutes.
When 5 minutes have passed, whisk in rice vinegar, soy sauce, and heavy cream. Turn burner heat on to medium, heat sauce just to a simmer. Allow to simmer gently for 5 minutes, as you prepare the tuna steaks.
Heat olive oil in a nonstick fry pan or skillet. Gently transfer tuna steaks to the pan, leaving at least 1″ space between each. Cook for a minute and a half on the first side, then flip and cook a minute and a half on the second side. This will give you a nice rare tuna steak – cook for longer if you prefer it to be more cooked.
Slice tuna steaks, fan them out, and serve with wasabi cream sauce.
* If you don’t have wasabi powder, use ~ 2 Tbsp prepared wasabi, omitting the water.