Step 1: In a small saucepan, combine the mixed berries, 1/2 cup water, and granulated sugar. Bring to a gentle simmer over medium heat, stirring occasionally, until the sugar has dissolved and the berries have softened, about 5-7 minutes.
Step 2: Remove the saucepan from the heat. Add the fresh basil leaves to the hot berry mixture. Stir gently and let it steep for 10-15 minutes to infuse the basil flavor.
Step 3: Strain the mixture through a fine-mesh sieve into a clean jar or container, pressing gently on the solids to extract all the liquid. Discard the solids. Let the berry-basil syrup cool completely. This syrup can be stored in the refrigerator for up to a week.
Step 4: To assemble each mocktail, fill a tall glass with ice.
Step 5: Add 2-3 tablespoons of the cooled berry-basil syrup to the glass, adjusting to your desired sweetness.
Step 6: Squeeze in 1-2 tablespoons of fresh lime juice.
Step 7: Top off the glass with sparkling water. Stir gently to combine all the ingredients.
Step 8: Garnish with a few fresh berries, a sprig of basil, and a lime wedge. Serve immediately and enjoy!