Using a mortar & pestle, spice grinder, or a blender, crush the peppercorns. Add to the sesame seeds and mix together. Spread out on a rimmed baking sheet or a plate.
Lay the tuna steaks into the mixture and use your fingers to press into all sides so they are coated.
Heat 1 tbsp olive oil or any neutral oil in a pan until it's sizzling hot. You are going to flash sear the tuna so the pan needs to be REALLY hot.
Gently place the crusted steaks in the oil, cook for 1.5 minutes on the first side, then flip and cook another 1.5 minutes. These will be beatifully rare in the center and cooked on the edges, but if you like it more well done cook for longer. Remove to a cutting board.
In a high powered blender, combine the cashews and 3/4 cup water and blend until smooth and creamy, about 2 minutes. If not using a high powered blender, be sure to soak your cashews in water for 2 hours before blending.
In a small sauce pan (NOT over heat), whisk the wasabi powder into 1 tbsp water until smooth. Let stand for 5 minutes. If using prepared wasabi instead of wasabi powder, skip this step and proceed to the next.
Whisk in coconut aminos, rice vinegar, and cashew cream.
Place sauce pan over medium heat and bring sauce to a simmer. Let simmer (NOT boil) for about 5 minutes until heated through, whisking constantly. Add additional water to thin the sauce to your liking.
Slice the tuna steaks and serve with the sauce!