Start by soaking the rice (see headnotes). Drain and rinse well, then place in a cooking pot with 2 ½ + cups / 625ml + water, plus the salt. Bring to a boil, reduce to simmer, and cook covered for about 35-40 minutes if soaked, 45-60 minutes if cooked from raw. Check the water levels periodically to make sure the pot isn’t drying out, and add water to the pot if necessary. When the rice is cooked, remove the lid and let cool for a few minutes. Add the mirin, and fold to combine.
Prepare a standard brownie pan (8x8” / 20x20cm) by rubbing it with a little fat to prevent sticking (olive oil, expeller-pressed coconut oil, ghee, or butter). Press the rice firmly into the pan, making it as level and even as possible. Place in the fridge for at least 4 hours, or overnight.
Make the chaviar by combining the nori with water from a recently-boiled kettle. Let soak for 1-2 minutes, then add the tamari and transfer this mix in a blender (or use an immersion blender). Blend on high until smooth. Transfer mixture to a jar, then stir in the chia seeds. Let the chia absorb the liquid, stirring occasionally. Set aside.
Make the kimchi mayo by stirring the ingredients together in a small bowl.
Scrub the carrots well, and chop them into your desired size - just make sure that they are relatively similar and bite-sized so that they cook evenly. Place in a steamer basket in a pot with water and set to medium-high heat, cover, and cook for 4-8 minutes once steaming - depending on their size and your preference. While the carrots are steaming, whisk together the tamari, lemon juice, olive oil, and grated ginger in a medium bowl and set aside. The carrots are ready when they are tender. Remove from heat, and immediately add to the bowl with the marinade. Stir well to coat. Let cool.
While the carrots are cooling, cut the rice out into your desired shapes. I used a jar lid (see photo) that would create four equal-sized portions, but you can also just cut the rice slab into four squares as well (alternatively, make a bunch of small, bite-sized pieces for appetizers!).
Place the rice bases on your plates. Spread a dollop of the kimchi mayo on top of each base and spread it to the edges. Top with avocado slices, then a few spoonfuls of the chaviar on top. Fold the remaining mayo through the marinated carrots, then spoon those on top of the avocado. Sprinkle with finely sliced spring onions and sesame seeds. Say thank you and enjoy immediately.