In a large saucepan add the oil, onions and a pinch of salt and gently cook for around 10mins until the onion are soft, but not brown.
Add the redcurrant jelly and mustard and stir until the jelly has melted and combined into the onions.
Stir in the flour, once all mixed together, slowly add the the port, stirring continuously until there are no lumps. Add the orange and lemon juice and the chicken stock, stirring until all combined.
Reduce the heat to low, cover with a lid and gently simmer for around 15mins, stirring occasionally.
Taste and season before serving.