Step 1: In a small saucepan, combine the sliced strawberries, granulated sugar, and water. Bring the mixture to a gentle simmer over medium heat, stirring until the sugar dissolves.
Step 2: Reduce heat to low and continue to simmer for 10-15 minutes, or until the strawberries are very soft and have released their juices, forming a syrupy consistency.
Step 3: Remove the saucepan from the heat and let the mixture cool slightly. Use a potato masher or a fork to mash the strawberries directly in the saucepan. For a smoother syrup, blend the mixture until smooth and then strain through a fine-mesh sieve into a clean jar, pressing on solids to extract all liquid. Discard solids. Let the syrup cool completely.
Step 4: To assemble each cooler, fill a tall glass with ice.
Step 5: Add 2-3 tablespoons of the cooled strawberry syrup to the glass, adjusting to your desired sweetness.
Step 6: Squeeze in about 1/2 tablespoon of fresh lemon juice.
Step 7: Top the glass with chilled sparkling water or club soda. Stir gently to combine.
Step 8: Garnish with a fresh strawberry slice, lemon wheel, or mint sprig, if desired, and serve immediately.