Purée the raspberries and strawberries. Press through a sieve. Add the sugar and lemon juice. Chill.
To serve, stir the mixed fruit into the purée. Spoon into soup plates. Scatter over the mixed fruit and mint. Set the crème fraîche on the table so everyone can add a dollop. Pass the bottle of Champagne and let everyone pour a glug into his bowl. Pour the rest of the bubbly into glasses.