Japanese Sushi Roll (Makizushi) With Canned Tuna

Ingredients (10)

• 1 tablespoon finely chopped onion
• 2/3 cup canned tuna
• 1 tablespoon mayonnaise
• 1 dash salt
• 1 teaspoon Japanese hot mustard (karashi), optional
• 4 sheets nori (dried seaweed)
• 6 cups cooked sushi rice
• Gari shoga (sweet pickled ginger), for garnish
• 2 teaspoons toasted and/or black sesame seeds, optional
• Soy sauce, for serving