1. Make a simple syrup by adding sugar and 1 cup water to a saucepan. Bring to a boil over low heat and stir until the sugar dissolves. Remove from the heat and set aside.
2. Pour cold water, lemon juice, lemon slices and simple syrup into a large pitcher and stir well.
3. Pour lemonade into glasses, filling about 1/2 to 2/3 the way full.
4. Top off each glass with sparkling wine and garnish with curls of lemon peel or additional slices of lemon.