Ahi Tuna Stacks with Ginger-Soy Dressing

Make the dressing: Whisk together all of the dressing ingredients in a small bowl then set it aside. Assemble the tuna stacks: Place the ring mold on the plate on which you're going to serve the tuna stack and then layer one fourth of the diced cucumber, mango, avocado, tuna and red onion inside the mold, pressing the ingredients firmly on top of one another. Press down on the top layer while slowly removing the mold to reveal the tuna stack. Repeat the layering process with the remaining ingredients to form three more tuna stacks. Drizzle each stack with a portion of the ginger-lime dressing, sprinkle with the toasted sesame seeds and serve immediately.