Bring 1.25 litres (5 cups) of water to the boil in a medium pot. Rinse sushi rice 3-4 times until the water runs clear. Add rice and dried wakame to boiling water. Cook uncovered on high for 10 minutes, then reduce the heat to low for an additional 10 minutes. Drain. For cold rice, rinse under cold water. For warm rice, proceed immediately.
Make rice dressing: Combine rice wine vinegar, salt, and sugar in a small pot. Bring to the boil, stirring until dissolved. Mix with the cooked rice (hot or cold).
Cut salmon into 2 cm (¾-inch) cubes. Combine with sesame oil, soy sauce, honey, sesame seeds, and spring onions. Mix gently and let marinate for 10 minutes whilst you prep other components.
Divide seasoned rice between two bowls. Top with marinated salmon, avocado cubes, pickled ginger, and garnishes. Drizzle with wasabi mayo.
Serve immediately with extra soy sauce on the side.