Prepare the pan: Line an 8-inch square pan or similarly sized container with parchment paper, leaving enough overhang to lift the rice later.
Press the rice: Add the warm cooked rice and press it firmly into an even layer approximately ½ inch thick. Compact it well so the pieces hold together.
Chill: Cover and refrigerate the rice for at least 1 hour or overnight.
Cut the rice: Lift the chilled rice from the pan. Cut it into 8 equal rectangles using a sharp, lightly wet knife.
Heat the oil: Add 2 tablespoons neutral oil to a large nonstick skillet and heat over medium heat.
Fry the first side: Add the rice pieces in a single layer. Work in batches when necessary. Cook without moving them for approximately 3 to 4 minutes.
Turn: Once the bottoms are deep golden and release easily, carefully turn the rice pieces.
Crisp the second side: Cook for another 3 to 4 minutes, adding a little more oil when the pan looks dry.
Drain: Transfer the crispy rice to a wire rack or paper towel-lined plate.
Drain the tuna: Press as much liquid as possible from the canned tuna, then flake it into a medium bowl.
Make the spicy tuna: Add the mayonnaise, sriracha, sesame oil, white sesame seeds, and black sesame seeds. Mix until creamy and evenly combined.
Taste: Add more sriracha when you want a stronger spicy flavor.
Add the avocado: Place thin avocado slices over each warm crispy rice piece.
Add the tuna: Spoon the spicy tuna mixture over the avocado.
Garnish: Finish with black sesame seeds, sliced jalapeño, and additional sriracha when desired.
Serve immediately: Arrange the bites on a platter and serve while the rice is still crisp.