Add raspberries and mint to a mixing glass and muddle gently to release juices and oils.
Add lime juice and simple syrup; stir to combine.
Fill a tall glass with ice and strain the raspberry–mint mixture into the glass.
Top with club soda or sparkling water and stir gently to combine.
Garnish with extra mint or a lemon slice. Serve immediately while fizzy.
Notes
Use fewer raspberries or less simple syrup for a drier drink. Prep the raspberry–mint base ahead and keep chilled; add soda at the last minute to keep the bubbles lively. If using dried mint, skip muddling and use a small pinch instead.