Prepare the Rice: Cook the short-grain sushi rice according to the manufacturer's instructions or using a rice cooker. While the rice remains warm, season it with a mixture composed of rice vinegar, granulated sugar, and a small pinch of salt. Stir gently to combine, then set aside to allow it to cool slightly.
Cook the Shrimp: In a non-stick skillet, heat the sesame oil over medium heat. Add the freshly grated garlic and ginger, sautéing briefly until aromatic. Introduce the shrimp to the skillet and cook for approximately 2 to 3 minutes on each side, or until they appear pink and opaque. Subsequently, add the soy sauce and sriracha sauce, stirring thoroughly to ensure the shrimp are evenly coated. Remove from heat and allow the shrimp to cool slightly.
Assemble the Rice Mixture: In a large mixing bowl, gently combine the seasoned rice with the cooked shrimp (reserving several shrimp for garnish, if desired), toasted sesame seeds, and chopped green onions. Carefully mix the ingredients to achieve an even distribution without damaging the rice grains.
Form the Sushi Stacks: Using a ring mold or a food-safe stacking mold, firmly press the rice and shrimp mixture into the mold, forming compact, cylindrical stacks. Gently remove the mold to reveal the formed stacks. Repeat the process for the remaining mixture.
Garnish and Serve: Top each sushi stack with the reserved shrimp. Sprinkle furikake seasoning over each stack and garnish with additional finely chopped green onions. Optionally, accompany the stacks with thin slices of avocado, cucumber, or radish for added freshness and visual appeal. Serve immediately while warm, or refrigerate for up to one hour prior to serving to enjoy a chilled version.