Brew green tea, ideally sencha, and chill it completely — hot tea brewed and cooled tastes better than cold-brewed for this.
Fill a highball glass with hard ice and stir to chill, pouring off any melt.
Add the Japanese whisky and stir briefly.
Top with the cold green tea, pouring gently down the side.
Stir once and serve with no garnish.
One of the standard izakaya "wari" builds alongside mizuwari and oyuwari, usually made with shochu and just as good with whisky. Brew the tea a little stronger than you would drink it, since the ice will take the edge off. It handles salty food better than anything else in this list.
My Bar Shelf reads your shelf and shows you every cocktail you can make from what you already own, including Ocha-Wari.