Put sugar and 1 cup water into a small saucepan and brin to a boil. Cook for 1 minute until sugar has dissolved. Turn off heat and allow sugar syrup to cool.

In a pitcher, combine cooled syrup with lemon juice and remaining 2 cups of water and mix well. Fill a Champagne flute halfway with the lemonade and top with champagne. Garnish with a lemon slice.