Combine sugar and water in a saucepan and bring to a boil. Simmer for about five minutes, until sugar is fully dissolved, then add the mint, lemon juice, and berries. Remove from heat and let cool. Pour into a carafe and refrigerate until chilled.
When ready to serve, pour in the sparkling water. Pour over ice into glasses garnished with additional berries.
You can strain out the mint and berries before adding the sparkling water, but I like to leave them in - the flavor only intensifies!