Add whiskey, peach liqueur, and fresh lemon juice.
Shake for about 10 seconds until well-chilled.
Strain into an old-fashioned glass filled with fresh ice.
Slowly top with Mountain Dew to preserve carbonation, then give one gentle stir.
Garnish with a lemon slice or peach wedge and serve immediately.
Notes
Fresh lemon juice keeps the sweetness in check. For a lighter sip, use diet Mountain Dew or reduce the whiskey to 1½ oz. Prefer it stronger? Skip the peach liqueur and add a little more whiskey. Serve right away while it’s crisp and fizzy.