The day before, put the sparkling wine, grape juice, Fragolina and half a lemon in the fridge. Pour the grape juice, along with the sugar and the two teaspoons of lemon juice, into a large glass jug (preferably also chilled). Stir well to completely dissolve the sugar. Add the Fragolina and stir again, mixing all the ingredients together. Add the Spumante, tilting the jug a little and pouring slowly down one side (to avoid the formation of too much froth). Stir delicately and wait for the froth to disperse, then pour immediately into well chilled flute glasses.