Bring 150 ml water, lemon juice and sugar to the boil in a saucepan. Dissolve the sugar while stirring, let it cool down. Select the raspberries, puree them and pass them through a sieve. Stir the puree, syrup and champagne.
Pour into a flat mould or onto a tray and freeze. As soon as the mixture becomes firm at the edge, loosen and stir in. Repeat the process several times, then freeze for 2-3 hours. Take out, scrape off and arrange immediately in pre-cooled glasses