Combine 0.5 cup water and 0.5 cup granulated sugar in a small saucepan. Add the sliced half jalapeño (remove seeds for milder heat). Bring to a gentle simmer over medium heat, stirring until sugar dissolves. Reduce heat and simmer 3 minutes to infuse. Remove from heat and let cool to room temperature, then strain out jalapeño slices into a clean jar. Refrigerate. Syrup will keep 1–2 weeks.

Prepare garnishes

Cut a fresh pineapple wedge and a lime wheel. If you like extra heat, reserve one thin jalapeño slice from the syrup step to float on the finished drink.

Cocktail

Fill a cocktail shaker with ice (about 1 cup). Add 3 oz fresh pineapple juice, 0.75 oz fresh lime juice, 1.5 oz light rum, and 0.5 oz jalapeño syrup.

Shake vigorously for 12–15 seconds until the shaker is well-chilled and slightly frosted.

Fill a highball or Collins glass with ice. Strain the shaken mixture into the glass.

Top with 2 oz tonic water. Give the drink one gentle stir to integrate tonic without losing carbonation.

Garnish with the pineapple wedge and lime wheel; optionally float the reserved jalapeño slice on top for aroma and extra heat.

Serve immediately.