Red Onion Marmalade is one of those retro things, beloved of late 70's and early 80's Bistro's. You know the sort of place: black and white photographs of Greta Garbo, candles dripping wax stuffed into old wine bottles, rickety tables with red and white check tablecloths, 30's dance music on the hi-fidelity system hidden beneath the bar.
Come to think of it, there aren't many of these places left now in London. I can think of Grumbles in Pimlico (a relic of the 60's, I think), and that Polish/Mexican place in Shepherd's Market, but that's about it.
Onion Marmalade goes perfectly with terrines, and pates; which is why, of course, it was a staple of the 70's bistro. It's very easy to make, and keeps well. Here's my own version:
Slice up some red onions thinly, and cook them in a pan with some butter, and some sugar. You want them to caramelise, so let them cook right down. The sugar will help them to go a dark brown colour.
Next add a teaspoon of red wine vinegar, some mustard seeds, a teaspoon of redcurrant jelly, and a generous dash of port. Let the "marmalade" bubble away, until the liquid has more or less evaporated. Season with generous amounts of salt and black pepper.