Wash and prepare the cornelian cherries
Place the cherries in a large pot with the water. Bring to the boil, then simmer gently for 20 minutes until soft and skins split.
Mash lightly with a potato masher to release more juice.
Strain through a jelly bag or muslin-lined sieve. For clear cordial, leave overnight without pressing; for more yield, gently press.
Measure the strained juice and return it to a clean pan. Add 600–700 g sugar per litre of juice, along with the lemon juice.
Stir gently over low heat until the sugar dissolves, then bring just to the boil.
Pour hot cordial into sterilised bottles, leaving 2 cm headspace. Seal immediately.
Store sealed bottles in a cool, dark place for up to a year. Once opened, refrigerate and use within 2–3 weeks.