Combine rice vinegar, sugar, and salt in a small bowl; stir until dissolved and fold gently into the warm cooked sushi rice.
Line a 4x6 inch rectangular container with plastic wrap. Press the seasoned rice firmly into the container to a thickness of 0.75 inch. Cover and refrigerate for at least 2 hours or until very firm.
Remove the chilled rice block and cut into 12-16 uniform rectangles using a moistened sharp knife.
In a mixing bowl, combine the minced tuna, Kewpie mayo, Sriracha, sesame oil, and scallions. Mix thoroughly and refrigerate until assembly.
Heat vegetable oil in a non-stick skillet over medium-high heat. Fry the rice blocks in batches for 2-3 minutes per side until golden brown and crispy.
Drain the fried rice cakes on paper towels to remove excess oil.
Spoon approximately 1 tablespoon of the spicy tuna mixture onto each crispy rice block.
Garnish each piece with a jalapeño round and a sprinkle of black sesame seeds. Serve immediately.