To make the Yorkshire pudding batter: sift the flour into a large bowl. Break the egg into the center of the flour. Mix the water and the milk together in a jug. Pour the mixture slowly onto the flour and egg. As you start to pour the water/milk slowly, beat the mixture together with a whisk. Add the salt and continue to beat. The puddings will be lighter if the batter includes a little air
Once all the ingredients have been beaten together leave to stand, covered by a cloth, for 40 minutes
Meanwhile, peel the onions, and then boil them in water until they are soft. Allow them to cool and chop into small pieces
Put the chopped onions into a large bowl. Add the sage, parsley and mint, bread crumbs, black pepper and salt. (S&P to taste) Blend together mixture.
Slowly blend the herb-onion mixture into the Yorkshire pudding batter making sure that it is all well mixed.
Heat the oil in an ovenproof dish or roasting tray and pour over the herbed batter mixture.
Bake in the center of the preheated oven for 40 minutes until golden brown and well risen.