Take your raw salmon and marinate it in equal parts salt and sugar. (I use about 2~3 kg salt and sugar combined, for every 1 kg salmon.) Thinly slice your onion and let it sit in a bath of ice water.
Let your salmon marinate for 6 hours. Wipe off the excess sugar and salt with a towel, and slice. Prepare your vinegared sushi rice, and place the salmon on top.
Squeeze just a bit of mayo on the tops of each piece of sushi. Squeeze out excess water from the onions. Place a pinch of the sliced onions on top of each small mound of mayo. Sprinkle on sesame seeds as desired to finish.
Story Behind this Recipe
I think marinated salmon works a lot better as sushi than when it's simply raw and unseasoned.