Add mirin, sake, soy sauce, grated garlic, sugar, sesame oil to a bowl and mix until sugar dissolves. Set aside.
Cut tuna into ½-inch cubes. Place tuna and add half of sauce in a bowl and mix. Cover with wrap and place in the refrigerator for 25 mins to marinate.
Add Thai chili to remaining sauce and mix well. Set aside. In a bowl, scoop out avocados, lightly mash with a fork and season with kosher salt.
Layer avocado then tuna into a fold mold. Carefully remove food mold and top with remaining sauce. Garnish with sesame seeds and serve with fried wontons.