Rinse the rice several times until the water runs clear. Drain in a sieve. In a pot, combine the drained rice, salt, and warm water. Bring to a boil, stirring occasionally. Reduce the heat to medium, cover, and cook for 10 minutes. Turn off the heat and let the rice sit, covered, for an additional 12 minutes.
Heat rice vinegar, sugar, and salt in a small pan until dissolved.
Once the rice is ready, stir in the seasoning while it is still warm to make it sticky.
Use a sharp knife to cut the fish into thin slices against the grain.
Use a ring mold or cutter. Place ½ to ¾ teaspoon of sticky rice in the mold and press firmly. Add a slice of tuna, another layer of rice, a slice of salmon, more rice, and top with grouper. Press down firmly and remove from the mold.
Slice the avocado into thin strips and fan them out. Use the mold to cut the avocado slices and place them on top of each sushi piece. Garnish with tobiko and chives.