Bring a medium pot of water to a boil on the stove.
Add chopped potatoes and boil about 10-15 minutes until cooked through and tender.
Heat a cast iron or heavy bottomed skillet over medium heat.
Pour 1/4 cup ponzu over tuna steaks and let set for about 5 minutes while the pan heats up.
Remove the tuna from the marinade and press sesame seeds onto both sides of the steaks.
Pour sesame oil in hot skillet and swirl to cover the bottom.
Lay tuna steaks in pan and sear for 30 seconds, then flip and sear another 30 seconds.
Remove from pan and let rest for a couple minutes.
Pour the potatoes into a mixing bowl and add butter and cream cheese.
Combine with a mixer (or masher) until incorporated and then add 1-2 tbsp of wasabi paste (or more depending on hot spicy you want it).
Spoon a rounded mound of potatoes on 2 plates.
Slice the rare tuna steaks into thin slices and lay over top of the potatoes.
Dot four wasabi circles on the side of the plate and serve with a side of 1/4 cup ponzu for dipping on each plate.