Prepare a shrub by covering raspberries with sugar in a glass jar. Close the jar and shelve for two days at room temperature, while shaking it every now and then. Add apple cider and shelve for two more days, while shaking it every now and then. Strain the shrub through a sieve to obtain syrup.
Create foam by combining 100 ml Rabarcello, cava, egg white and 20 ml lemon juice in a whipping siphon. Charge, shake well and refrigerate for one hour. Shake 60 ml Rabarcello over ice with 20 ml shrub-syrup and 25 ml lemon juice. Strain into a chilled coupe and top off with foam. Garnish with rhubarb.