Melt butter in a large saucepan. Cook the onions over a medium heat in the butter until caramelised - about 15 minutes - stir regularly and make sure they don't burn.
Pour hot water over the currants and let drain, then add the currants and grated apple to the onions.
Add the wine, pomegranate molasses, vinegar and sugar.
Simmer for about 1 hour over a medium heat until the excess liquid has boiled off. Stir in the salt and pepper in the last 10 minutes.