Rinse 300g sushi rice under cold water until the water runs clear. Cook with 360ml water. Once cooked, fold in 60ml rice vinegar, 2 tbsp sugar, and 1 tsp salt while still warm. Fan as you fold to make it glossy.
Slice 200g sushi-grade salmon or tuna into 1cm strips. Cut 1 cucumber and half an avocado into thin strips. Lay out all fillings on a board.
Place a bamboo mat on your work surface with a sheet of nori on top, shiny side down. Wet your hands and spread a thin, even layer of rice over three-quarters of the nori, leaving a 2cm border at the far edge.
Lay your chosen fillings in a line across the centre of the rice. Using the mat, roll the nori tightly away from you, pressing as you go. Squeeze gently to form a firm cylinder. Seal the border with a dab of water.
With a wet, sharp knife, cut each roll into 8 pieces using a single confident stroke per cut. Serve with soy sauce, pickled ginger, and wasabi.