Heat the oil, first of all, in a heavy 18cm frying-pan then over a medium heat fry the chipolatas and onion rings togrether for about 10 minutes. Meanwhile preheat the grill and make up the batter.
To do this you sieve the flour into a bowl, make a well in the centre and break the egg into it. Now whisk the egg gradually incorporating the flour and finally whisk in the milk, until you have a smooth lump-free batter (you may need to stir round the edge of the bowl and have another whisk to make sure it's all blended). Then season with salt and pepper, and whisk in the herbs.
By this time the chipolatas will be ready, so increase the heat to its hightest setting, and when it is very hot pour in the batter all round the chipolatas. Then leave it, without stirring or anything, for 1 or 2 minutes or until it looks on the point of bubbling up slightly here and there.
It will look a total disaster at this stage, but all is well. Just transfer the pan to sit under the grill (allowing room for the batter to rise) and let it finish cooking for about 6-10 minutes, or until it has become beautifully puffy and very crisp and brown. Now serve immediately-it will shrink somewhat but remain light and crisp.