In a small bowl, mix rice vinegar, sugar, and salt until dissolved. Pour over the cooked sushi rice and gently fold until evenly seasoned.
Line a baking pan with plastic wrap and press the rice into an even layer. Cover and refrigerate for at least 4 hours or overnight to firm up.
For the topping, combine chopped salmon, Kewpie mayo, sriracha, soy sauce, scallions, and sesame oil in a bowl. Mix well and refrigerate until ready to use.
Once chilled, cut the rice into 16 rectangles. Heat vegetable oil in a skillet over medium heat and fry the rice until both sides are crisp and golden.
Drain on paper towels and allow to cool slightly.
Assemble by topping each crispy rice piece with avocado, a spoonful of spicy salmon, jalapeño slices, and a sprinkle of sesame seeds. Serve immediately.