To a small mixing bowl, add the buttermilk, sour cream, mayonnaise, sriracha, lemon juice, sesame oil, soy sauce, rice vinegar, and sugar and mix until combined.
In another bowl, add the cucumber sticks, vinegar, sugar, sesame oil and salt and pepper to a bowl. Toss to combine.
To serve, to each bowl add the carrots, the cucumber and avocado tuna and rice. Then drizzle over the spicy mayo or mix into the tuna.
Add the optional toppings if you like.