Mix sugar with 200 ml (approximately 7 ounces) of water in a saucepan, simmer for about 15 minutes and then cool. Thaw raspberries, put some nice berries aside. Puree remaining berries with lemon juice in a blender, pour through a fine sieve and then mix in a bowl with sparkling wine and sugar syrup. Pour everything into a shallow metal bowl. Freeze for at least 4 hours, stirring often with a fork to prevent the formation of large ice crystals. (Or finish in an ice cream machine).
Serve sorbet in small bowls garnished with raspberries.