Preheat oven to 200˚C. Halve each pepper and remove seeds. Place the pepper halves skin-side down on a well-greased oven dish and roast until the skin is blackened and peeling. Remove and allow to cool before peeling off the skin. Lightly sauté the onions in some olive oil until soft. Place in a blender with the peeled peppers, red wine vinegar, salt and pepper. Blend together until smooth, then pass the mixture through a strainer to remove any unblended bits. For a creamier dip, or to make it go a little bit further, fold through some smooth cream cheese.
Tip: To add a bit of spicy heat to your dip, roast a chilli or two with the peppers, and prepare in the same way as the peppers.