First, make the pickled onions. Slice the red onion into strips. Place in a mesh sieve. Set aside. Add 3 cups of water to a medium saucepan. Place on medium high heat to boil the water.
Once the water is boiling, turn off heat. While over the sink, slowly pour hot water over sliced onions in mesh sieve. Place onions in medium mason jar. Add vinegar, salt, sugar, and garlic. Shake to combine ingredients. Place in fridge to sit for at least one hour.
Whisk all the ingredients for the poke sauce. Add cubed raw tuna and toss to combine. Once the onions have pickled for at least an hour. remove from fridge to prepare for stack assembly.
Using a round mold, prepare to stack you ingredients by placing mold in center of a plate. Scoop 1/2 cup rice and place into bottom of mold. Press down to pack the rice in firmly. Next, arrange sliced picked onions into one even layer. Then, add cubed avocado into an even layer. Finally, top with ahi tuna poke.
Now that the stack is assembled, gently remove the mold by holding the top ingredients in place while you lift the mold up. Finish the stack with a spicy mayo by combining the sriracha with the mayo and drizzling over the stack. Top with black and while sesame seeds and serve!