Step by step

1. Add the water, cranberries and caster sugar to a small pan and bring to simmer over low heat. Cook, stirring regularly for 15 minutes until cranberries are soft and the sauce has thickened.

2. Pour into the jug of a blender. Mix for 30 seconds on speed 6. Strain out the seeds with a sieve and cool for 30 minutes.

3. Add a tablespoon of the cranberry coulis to each glass, then add the ice cubes and top off with sparkling elderflower water. Serve with a sprig of rosemary.