Mix soy sauce, mirin and sugar together in small pan. Heat over medium-low flame and reduce by 25%. Remove from heat.
Mix cornstarch and enough water to make a thick slurry. Stir into soy sauce mixture. As the sauce cools it will thicken.
Heat nori sheets over an open flame for a few seconds per side (this adds flavor).
Place a bamboo sushi mat down with bamboo lines running horizontally. Then add one sheet of nori on the bamboo sushi mat (you can lay a piece of plastic wrap on the surface of the mat first if you want to protect it and to help avoid stickiness) and spread ¾ to 1 cup cooked sushi rice on the ¾ of the nori sheet closest to you.
Put cucumber strips, black cod and broccoli sprouts 1 inch from the start of the nori and rice closest to you, leaving 1½ inches of rice on the far side without ingredients and 1 inch of plain nori to seal the roll on the far end.
Start rolling from the loaded end and apply pressure as you go. Wet the end of the nori sheet and complete the roll.
Cut into 6 to 8 bite-size pieces and drizzle with unagi sauce.
Optional: serve with pickled ginger and wasabi. Enjoy!