Step 1

Heat the oil and butter in a large heavy-based casserole. Add the onions and fry for several minutes on a medium heat until starting to caramelise. Add all the remaining ingredients, then season with salt and pepper.

Step 2

Bring to the boil, then turn down and cover. Leave to simmer for around 30 minutes, stirring fairly regularly, then remove the lid and turn up the heat slightly.

Step 3

Continue to cook until the liquid has reduced and is starting to become syrupy. Check the seasoning and serve.