Rinse the Sushi Rice under cold running water until the water runs clear, then drain. Combine the rice and Water in a pot, bring to a gentle boil, reduce heat to low, cover and simmer until the water is absorbed (about 15–18 minutes). Remove from heat and let stand, covered, for 10 minutes.
While the rice cooks, combine Rice Vinegar, Sugar and Salt in a small saucepan and warm gently until the sugar and salt dissolve; cool slightly.
Transfer the cooked rice to a large, shallow bowl and gently fold in the vinegar mixture with a rice paddle or spatula, using a slicing motion to avoid crushing the grains. Fan the rice as you mix to help it become glossy and cool to near room temperature.
Prepare the fillings: slice the Sashimi-Grade Tuna Loin into long, thin strips and cut the Cucumber into julienne batons. Arrange them so they’re ready to use.
Place a sheet of Nori shiny side down on a bamboo rolling mat. Wet your hands to prevent sticking and spread an even, thin layer of rice over the nori, leaving about a 1-inch (2 cm) bare border along the top edge.
Lay a neat line of tuna and cucumber across the rice about one-third of the way up from the bottom, pressing gently so the fillings adhere to the rice.
Using the mat, roll the nori over the fillings tightly away from you, pressing to form a firm cylinder; wet the exposed nori edge and press to seal the roll.
With a very sharp knife dipped in water, slice the roll into 6–8 even pieces, wiping the blade between cuts for clean edges.
Arrange the maki on a platter and serve with small dishes of Soy Sauce, a dab of Wasabi Paste and Pickled Ginger on the side.
