Preheat the oven to gas 3, 160°C, 325°F.
Place onions on a baking tray, drizzle over 2tbsp of the oil and season. Bake for 1 hour and 15 minutes or so, until softened and beginning to caramelize around the edges.
In the meantime heat the rest of the oil in a pan, add the fennel seeds, chilli, raisins and sugar. When bubbling add the vinegar and pour over the onions. Season with salt and pepper and, if necessary, more vinegar.
Enjoy with roasts or cold meats.