Slice the tuna sashimi into strips that are 1/4 to 1/2 inch thick, depending on your preference. Mix the soy sauce, mirin, sake, sesame oil and dashi powder in a bowl and stir well. Add the tuna slices to the bowl and gently toss to ensure each piece is evenly coated. Let the tuna and sauce sit for 30 minutes. Divide your rice into two bowls and top with shredded nori. Then add the marinated tuna and top with chopped scallions and shiso leaves. Sprinkle a few sesame seeds on top. Pour an equal amount of any leftover marinade over the top of each bowl and serve immediately.