Make the dressing by combining the vinegars and seasoning and then vigorously whisking in the combined oils to make a 'creamy' emulsion. Chop the tomatoes very finely, add them to the dressing, and leave to stand on one side. Peel the onions and slice them into rings, no more than 1/4in thick preferably finer.
Put the oil and butter into a frying pan and heat over a moderate flame. Once the butter has finished bubling, add the onion rings, stirring round until they are all well coated. Cook for about 15 minutes, tossing and stirring frequently until the onions have softened and taken on a little colour. Remove the pan from the heat and set aside.
Slice the goats cheeses into halves. Turn the grill on high. Quickly dress the salad leaves with a little of the dressing and divide between 6 plates. Then place the cheese halves on a baking tray, cut side up, and put under for about 5 minutes, or until the tops are covered with golden brown freckles and the cheese inside has started to melt. Put the onions back over a high flame and, stirring continually, tip in the sherry vinegar and sugar. Bubble furiously for a couple of minutes until you can see the juices have turned syrupy. Remove from the heat and season lavishly with salt and pepper.
To serve, put a goat's cheese half on each plate with the salad and then strew with the sweet and sour onions. Finally, dress with another tablespoon of the dressing, making sure that each plate gets a good sprinkling of sun-dried tomatoes.