Prepare the Sushi Rice: Rinse the sushi rice under cold running water in a fine-mesh sieve until the water runs clear. This removes excess starch, which is crucial for the right texture. Transfer the rinsed rice to a medium saucepan and add the 600ml (2.5 cups) of water. Let it soak for 30 minutes. After soaking, bring the water and rice to a boil over medium-high heat. Once boiling, reduce the heat to low, cover tightly, and simmer for 15 minutes. Do not lift the lid during this time. After 15 minutes, remove the saucepan from the heat and let it steam, still covered, for another 10-15 minutes. This steaming process allows the rice to absorb any remaining moisture evenly.

Make the Sushi Vinegar Seasoning: While the rice is steaming, prepare the sushi vinegar. In a small saucepan, combine the 75ml (5 tbsp) rice vinegar, 25g (2 tbsp) sugar, and 5g (1 tsp) salt. Heat gently over low heat, stirring constantly, until the sugar and salt are completely dissolved. Do not boil. Remove from heat and let it cool.

Season the Rice: Once the rice has finished steaming, carefully transfer it to a large, non-metallic bowl (a wooden or plastic bowl is ideal, as metal can react with the vinegar). Gently spread the rice out with a rice paddle or wooden spoon. Gradually pour the cooled sushi vinegar mixture over the rice. Use a slicing motion with the paddle to mix the vinegar into the rice, being careful not to mash the grains. Fan the rice vigorously as you mix; this helps it cool quickly and gives it a glossy sheen. Continue fanning and mixing until the rice is room temperature.

Prepare the Fillings: Peel the cucumber and cut it into long, thin matchsticks, approximately 0.5cm (0.2 inch) thick. For the avocado, halve it, remove the pit, peel it, and slice it into similar-sized strips. If using fish, slice it thinly against the grain into strips about 0.5cm (0.2 inch) thick and long enough to span the width of the nori sheet.

Assemble the Maki Rolls: Place a bamboo sushi rolling mat (makisu) on a clean, dry surface. Place one sheet of nori, shiny side down, on the mat, with the longer side parallel to you. Have a small bowl of water mixed with a little rice vinegar (a 1:1 ratio of water to vinegar) nearby to wet your hands. Wet your hands to prevent the rice from sticking. Take about 1/4 of the seasoned sushi rice (approximately 150g or 1 cup) and spread it evenly over the nori sheet, leaving a 2.5cm (1 inch) border bare at the top edge furthest from you. The rice layer should be thin, about 0.5cm (0.2 inch) thick. Be gentle not to press down too hard.

Add the Fillings: Arrange your chosen fillings horizontally across the rice, about one-third of the way up from the bottom edge closest to you. Don't overfill, as this will make rolling difficult. For example, lay a few strips of cucumber, avocado, and a strip of fish.

Roll the Sushi: Using the bamboo mat, lift the edge closest to you and carefully roll it over the fillings, tucking the edge of the nori under to meet the rice on the other side. Apply gentle, even pressure with your fingers to form a tight cylinder. Continue rolling, using the mat to guide and shape the roll. As you reach the end, moisten the bare strip of nori at the top with a little water to help seal the roll. Complete the roll, pressing gently to secure its shape.

Cut the Maki Rolls: Transfer the rolled sushi, seam-side down, onto a cutting board. Use a very sharp knife that has been slightly dampened with water to prevent sticking. Cut the roll in half first. Then, cut each half into three equal pieces, resulting in 6 pieces per roll. Wipe the knife with a damp cloth between cuts for cleaner slices.

Serve: Arrange the sushi pieces attractively on a platter. Serve immediately with soy sauce for dipping, a small amount of wasabi paste, and pickled ginger (gari) to cleanse the palate between pieces.