The day before, boil up the sugar with 300 ml water. Wash 1 bunch of basil, shake dry, pluck leaves. Wash lemons hot, dab dry, peel the peel. Squeeze 1 lemon. Add the basil, lemon peel and juice to the sugar water and leave to stand overnight.
The next day, sieve the sugar stock and simmer at medium heat for about 10 minutes. Pour hot syrup into the bottle, close it and let it cool down. Syrup keeps refrigerated for about 2 months.
To serve, put about 2 tablespoons of syrup and basil leaves into champagne glasses, fill up with Prosecco.