In a medium saucepan, combine 1 cup of granulated sugar with 1 cup of cold water. Heat over medium-high, stirring occasionally until the sugar fully dissolves, about 5 minutes. Let it cool to room temperature.

Juice about 5-6 fresh lemons and strain the juice through a fine mesh sieve to remove seeds and pulp. Combine the strained lemon juice with the pulp from approximately 8 ripe passion fruits in a mixing bowl.

Once the simple syrup has cooled, add it to the lemon and passion fruit mixture. Slowly pour in 4 cups of cold water, stirring gently to combine, and adjust sweetness or tartness as needed.

Transfer the mixture into a large pitcher, cover it, and refrigerate for at least 1 hour to allow flavors to meld.

Before serving, stir well. Pour the drink over ice in tall glasses and garnish with lemon slices or whole passion fruits.