Place rice in a fine-mesh strainer and rinse under cold running water, gently agitating with your hands. Continue rinsing until water runs clear, about 5-7 rinses. This removes excess starch for proper texture.

Transfer rinsed rice to a medium saucepan and add 2.25 cups filtered water. Add kombu piece if using. Let rice soak for 15 minutes before cooking for better texture.

Bring rice to a boil over medium-high heat. Once boiling, immediately reduce heat to lowest setting, cover tightly with lid, and simmer for exactly 18 minutes. Do not lift the lid during cooking.

Remove from heat and let stand, covered, for 5 minutes. Remove and discard kombu piece if used.

While rice cooks, combine rice vinegar, coconut sugar, and sea salt in a small saucepan. Heat over low heat, stirring constantly, until sugar and salt completely dissolve, about 2-3 minutes. Do not boil. Set aside to cool slightly.

Transfer hot cooked rice to a large wooden or plastic mixing bowl (avoid metal as it can affect flavor). Never use a metal bowl for sushi rice.

Gradually drizzle the warm seasoning mixture over the rice while gently folding with a rice paddle or wooden spoon. Use a cutting and folding motion rather than stirring to avoid mashing the grains.

Continue folding while fanning the rice with a kitchen towel or fan to cool it to room temperature, about 10-15 minutes. The rice should have a glossy appearance when properly seasoned.