For the sauce combine Miso, wasabi, sriracha sauce and sesame oil in a small bowl.
Start mixing it all together adding mirin . Do not pour all mirin at once, you might end up having clumpy paste.
Add Ponzu sauce and soy sauce.
Mix everything together until all ingredients are blended and there is no clumps. Pour the sauce into a glass container. This sauce can be stored in the refrigerator for several months and can be used for other recipes as well.
Put tuna in a freezer for half and hour, this way it will be easier to slice it.
Pat tuna dray and cut it diagonally into 1/2 inch slices.
Cut each slice into 1/2 inch strips. Then cut strips into 1/2 cubes.
Pour sauce and mix well until all tuna is covered. Let it marinate in a fridge for about 10 minutes.
Using food ring plate your tuna tartar, dust it with sesame seeds and enjoy!