Over in Ebisu there's an izakaya called Ojinjo, famous for their lemon chūhais. In fact they have six different kinds of lemon chūhais on the menu, such as the "Grampa's Barley Lemon-hai," which uses barley shōchū and ultra-strong soda, or the "Limoncello de Hai" which swaps out shōchū entirely for limoncello. Lemon chūhai with crushed ice, or one that uses mint. Lots of variety!
Just in case you're wondering what a chūhai is. It's short for "shōchū highball," so basically shōchū + soda. A staple of the izakaya drink menu, chūhai are usually flavored with some kind of fruit juice. If you are in the market be sure to look for chūhai that say they are 生絞り (nama shibori, meaning fresh squeezed) sometimes shortened to just 生 -- this means you get real fruit juice, or even the fruit itself and and a juicer so you do it on your own. Lemon and ume (plum) are the most popular but you also sometimes see grapefruit, kiwi, pineapple, orange, etc.
In any case, Takara Shuzo, maker and supplier of affordable shōchūs to izakaya across the country, recently featured an article detailing what goes into Ojinjo's famous lemon chūhais. (Language note: the article refers to lemon chūhais as レモンサワー which certainly translates as "lemon sour" but calling the drink this in English can be very confusing. Let's stick with chūhai)
According to Watanabe-san, a good lemon chūhai follows the recipe below!