Rhubarb syrup: Take a baking dish and fill it with 1 part rhubarb, 1 part sugar and 1 part water. Let the mixture roast for 30 minutes in your oven (200° C / fan 180° C / gas 6). Then strain and allow it to cool. Put all the ingredients except the soda water into your shaker and dry-shake for about 10 seconds. Then shake very well with three or four ice cubes until the foam shows the desired consistency. Now strain into a chilled glass, top it up with soda water and garnish with rose petals. The dessert is ready to be served.