Add the elderberries, apple cider vinegar, brown sugar, and water to a pan and gently simmer for 20 minutes.
Crush the berries down with the back of a fork to release their juices.
Bring the heat up and reduce until the sauce thickens, about a quarter in volume, and takes on a sticky, thick texture.
Remove from heat and allow to cool slightly.
Pour the sauce through a strainer into a bowl and press down with the back of a spoon.
Top with olive oil and a pinch of salt.
