Place tuna, marinade and sesame seeds in mixing bowl. Stir and let marinate for 5 minutes. While marinating, assemble "tower" base with avocado and pineapple. Top with marinated tuna and pack tightly.
In a medium saucepan, heat sesame oil until smoking. Add the garlic, shallot, and ginger. Saute until garlic begins to change color. Deglaze pan with sake. Bring to a simmer and let reduce by half. Add vinegar, soy, and sugar. Return to a boil, blend and then strain into clean saucepan. Whisk in slurry and bring to a boil to thicken. Remove from heat. Chill before using.
Recipe courtesy of Robert Lia, Geoffrey's