Add rice, 1 cup water, and ½ tsp salt to a small pot. Bring to a boil, cover, and reduce heat to low. Simmer for 10–15 minutes until water is absorbed. Remove from heat, keep covered, and let steam for 10 minutes. Fluff with a fork, stir in rice vinegar, and let cool.
In a small bowl, toss diced mango, cucumber, and red onion with 1 tsp lemon juice.
Peel and pit the avocado. Mash in a bowl with 1 tsp lemon juice, salt, and pepper (I add 1/8 tsp of each.)
Drain tuna well and add to a bowl. Mix with ¼ cup mayo and 1 tsp lemon juice.
Lightly spray four 1-cup measuring cups with cooking spray.Layer 1: Divide mango salad evenly, pressing down with a spoon.Layer 2: Add avocado and smooth into an even layer.Layer 3: Add tuna and press down gently.Layer 4: Top with rice, pressing down firmly with the back of a spoon. Refrigerate stacks for a few minutes to make them easier to remove.
Run a sharp knife around the edges. Place a plate over the measuring cup, flip it over, and gently lift the cup off. If any mango salad sticks, scoop it out and place it on top. Drizzle each stack with 1–2 tsp soy sauce and sesame seeds if using.
Mix remaining ¼ cup mayo with Sriracha (see note 3) and 1 tbsp lemon juice. Drizzle over stacks and enjoy immediately.
Video
Notes
Note1: Alternatively, use cold leftover rice or microwave rice packets (1 cup uncooked) for a super-quick assembly.Note 2: Or use mashed avocado packs: Check the label to make sure the only ingredient is avocado.Note 3: Adjust Sriracha to your spice preference. If it’s too spicy, stir in a little sugar to balance the heat.Storage: Best enjoyed fresh. If you have leftovers, store each component separately and assemble just before eating.