Using a sharp knife, cut off the dark purple and discard (this part is called myoglobin and is a blood pigment, it's very strongly flavored.) Dice the tuna into 1/4 inch square pieces. Set aside.
In a small bowl, whisk the mayonnaise and sriracha together until combined. Set aside.
Divide the avocado among four small bowls. Top the avocado with equal amounts of tuna.
Drizzle the spicy sauce over the tuna and top each bowl with equal amounts of tobiko.