Cut apple into matchsticks with slicer, discarding core, then put in a pitcher and stir in soju.
Let macerate, covered and chilled, at least 30 minutes.
Fill 8 (12-ounce) double Old Fashioned glasses with ice. Strain about 1/4 cup soju into each glass, then add some apple pieces to each glass. Top off drinks with tonic and serve with lime wedges.