Make the sushi rice: Gently warm the rice wine vinegar in a heat-proof measuring cup in the microwave. Stir the sugar and salt into the warm vinegar until dissolved, set aside. Rinse rice in a fine-mesh strainer under cold water until the water runs clear. Transfer drained rice and 1 2/3 cups water to a rice cooker or Instant Pot. In a rice cooker, use the brown rice setting. In an Instant Pot, pressure cook on high for 15 minutes and naturally release pressure for 10 minutes before manually releasing the remaining steam.

Drizzle vinegar mixture over the rice, using a rice paddle to gently fold the rice to incorporate. Transfer to a bowl, cover lightly with food wrap, and cool to room temperature.

Make the spicy mayo: combine all ingredients in a small bowl and whisk until smooth. Transfer to a small squeeze bottle or piping bag for easy drizzling.

Make the poke: Combine the soy, mirin, sesame oil, and sriracha, and salt in a medium bowl and whisk to combine. Add tuna, salmon, sesame seeds, and scallions, tossing gently to combine. Refrigerate for 20 minutes. Alternatively, the two types of fish can be mixed separately (just divide the sauce in two) so that the colors remain vibrant and separate.

Assemble the bowls: start with some sushi rice in the middle of a shallow bowl. Top with the poke and any other desired toppings. Lightly drizzle with spicy mayo and top with additional sliced scallions and sesame seeds. Serve right away.