Prepare the Sushi Rice:
Cook sushi rice according to package directions
Let rice cool slightly until warm but not hot (easier to handle)
Keep rice covered to prevent drying
Make the Tuna Salad:
Drain canned tuna thoroughly, pressing out excess liquid
Transfer tuna to a bowl and break into small flakes with a fork
Add mayonnaise and lemon juice
If using, add finely chopped celery or onion
Mix everything together until well combined
Season with salt and pepper to taste
Set aside
Form the Rice Cups:
Lightly spray or brush a 12-cup cupcake pan with oil (or line with small pieces of plastic wrap)
Dampen your fingers with water to prevent sticking
Press sushi rice into each cupcake cup, creating a hollow center
Press rice firmly against bottom and sides to form a cup shape about 1/4 to 1/2 inch thick
Make sure the rice is packed firmly enough to hold its shape
Assemble the Cups:
Spoon 1-2 tablespoons of tuna salad into the center of each rice cup
Place a small slice of avocado on top of the tuna
Drizzle a small amount of sriracha sauce over each cup
Sprinkle generously with sesame seeds (black, white, or mixed)
Remove and Serve:
If using plastic wrap, lift each cup out using the overhanging edges
If oiled directly, run a knife around edges and carefully lift out with a spoon
Transfer to a serving plate
Serve immediately or refrigerate for up to 2-3 hours
Make-Ahead Tip: Prepare rice cups and tuna salad separately up to 24 hours ahead. Assemble and add toppings within a few hours of serving for best results. Add avocado just before serving to prevent browning.