Heat the oil in a large pan and slowly cook the onion for 6-8 mins to soften.
Meanwhile, peel the tomatoes: using a sharp knife, make a cross on the top and bottom of the tomatoes. Pour boiling water into a bowl, and carefully lower the tomatoes into the water. Prepare another bowl of very cold water. After about 30 secs, drain the tomatoes, then plunge them into the cold water. The skins should now slip off easily. Cut into quarters, deseed and roughly chop
Throw the peppers and garlic into the onion pan, give everything a good stir and cook, covered, over a low heat for 20 mins or so until the vegetables are really soft and sweet. Stir regularly so that they don’t catch on the bottom of the pan. Add the tomatoes, a pinch of sugar and cook, uncovered, for 5 mins more. Season and stir in the vinegar and the basil. Allow to cool to room temperature, stir through the basil and taste to check the flavours again. You may want to add a dash more vinegar, or a pinch more sugar, depending on how sweet your vegetables are. Spoon onto the baguette slices and serve.