Place all the ingredients in a saucepan. Cook the mixture for about 8 minutes stirring often. The fruit will become very soft and release lots of juice.
Remove from the heat and set aside to cool for at least 10 minutes then pour the mixture into a fine-mesh sieve and stir over a bowl to separate the pulp from the sauce. Serve warm or cold (see the post for serving ideas).
You do NOT need to remove the stems from the redcurrant before cooking.
Adjust the amount of sugar and balsamic vinegar according to preference and the kind of dish you are planning to serve it with.
Stir the sauce often as it cooks to prevent it sticking to the pan.
The sauce will be quite runny to start with but it will thicken as it cools so allow it to cool for at least 10 minutes before separating the pulp from the juice (it will have a better consistency).
Omit the onion granules if you want to serve this sauce in a dessert dish.
Serve warm or chilled.
Once cooled keep refrigerated for up to a week. If you've added butter into the sauce use it within 3 days.