Step 1Place koji in a medium non-reactive bowl or container and add sea sea.
Step 2Mix thoroughly and vigorously, using your hands.
Step 3Heat water to 140 degrees; add just enough to moisten.
Step 4Knead till fragrant. Mix in rest of hot water.
Step 5Store loosely covered for 1 week at room temperature. When ready, the koji rice grains will be smaller; mixture will have consistency of loose porridge. Refrigerate covered for up to 6 months.
Step 1Place the tuna in a large bowl and mix in the shio koji so that it's evenly distributed. Set aside.
Step 2Peel, seed and cut the avocados into a large dice and sprinkle with just enough lemon juice to coat so that it doesn't discolor. Add the diced avocado to the tuna mixture. Add dill and mix thoroughly. Divide the mixture into four bowls. Garnish each serving with a small sprig of dill and wasabi if desired. Serve immediately.