Whisk the flour, eggs, milk and a good pinch of salt in a jug until you have a smooth batter the consistency of double cream. Set aside to rest while the oven heats, or in the fridge for up to 24 hours.
Heat the oven to 220C (200C fan). Spoon a teaspoon of oil into 6 cups of a deep muffin tin and put the tin in the oven for 8 to 10 minutes until the oil is shimmering hot.
Give the batter a final whisk, then divide it between the hot cups, filling each about half full. Work quickly so the oil stays hot. Bake for 20 to 25 minutes without opening the oven door, until the puddings are puffed, golden and crisp.
While the puddings bake, make the gravy. Melt the butter in a frying pan over a medium heat, add the onions and a pinch of salt and cook for 10 minutes until soft and golden at the edges.
Stir in the flour, cook for a minute, then crumble in the stock cube and add 300ml of hot water and the Worcestershire sauce. Simmer for 5 minutes until it thickens to a glossy gravy that coats the spoon.
Serve two puddings per person in a shallow bowl with the onion gravy poured over and plenty of black pepper.