Divide the rice among 4 bowls. For each bowl, top with one-fourth of the nori. Lay one-fourth tuna slices over the nori Sprinkle one-fourth of the scallion over the tuna. Pile one-fourth of the shiso over the scallion. Drizzle 2 tablespoons of the marinade over the shiso. Repeat with the remaining bowls, and serve.