method
1. Preheat the oven to warm 160°C (315°F). Bake the onions in a baking dish for 1 1/2 hours. Leave until cool enough to handle.
2. Trim off the stems and peel away the skin (the outer part of the root should come away but the onions will remain intact). Rinse a 1 litre wide-necked jar with boiling water and dry in a warm oven (do not dry with a tea towel). Put the onions in the jar.
3. Combine the vinegar and sugar in a small screw-top jar and stir to dissolve the sugar. Add the oil, seal the jar and shake vigorously until combined—the mixture will be paler and may separate on standing.
4. Pour the vinegar mixture over the onions, seal, and turn upside down to coat. Marinate overnight in the refrigerator, turning occasionally. Return to room temperature and shake the jar to thoroughly combine the dressing before serving.
