1. Put cut up fish into a bowl with soy sauce and lemon juice and let it marinate for 10 minutes or up to an hour.
2. Thinly slice leeks and sauté in oil. Add salt to the leeks. Start the leeks on a higher heat, then lower the heat and cover. Check on the leeks after about 10 minutes. If there is excess liquid, uncover and raise the heat a little until the leeks are caramelized.
3. Thinly slice avocado to the same size as the fish pieces.
4. Mix half a cup of mayonnaise with a teaspoon of schug (or more or less according to your taste).