Pre-chill a highball glass in the freezer for at least 10 minutes.
Fill the chilled glass with large ice cubes, packing them tightly.
Pour 45ml of blended Scotch whisky directly over the ice.
Slowly pour 90ml of chilled soda water down the side of the glass to preserve carbonation.
Stir gently once with a bar spoon - just enough to combine.
Garnish with a lemon slice wheel on the rim and serve immediately.
The Japanese kissaten-style highball technique - using very cold, tightly packed ice and chilled soda poured slowly down the spoon - is widely considered the gold standard for this drink, minimizing dilution while maximizing effervescence.
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