In a saucepan, sweat off the onions in the olive oil for 10-15 minutes.
Add the mustard seeds and season.
Add the garlic, followed by the remainder of the ingredients, and stir.
Gently stew down for 1 hour or more until a jammy consistency.
Adjust the acidity – adding a touch more sugar if necessary – and season accordingly.
Recipe courtesy of Matt Tebbutt