Start by preparing the quick pickles. Slice the cucumber and mix it with salt, sugar, and vinegar. Let it sit to marinate.
In a bowl, combine the canned tuna with Kewpie mayo, adding salt and pepper to taste.
Prepare your bread for the melt. Spread the tuna mixture on one slice, and top it with avocado if desired.
Drizzle some rice vinegar and sriracha over the tuna for added flavor.
Top with another slice of bread and grill until golden brown on both sides.
