For the Yorkshire puddings, to make the batter, place the flour in a large bowl and season with salt and pepper. Add the eggs and whisk. Gradually add the milk, whisking all the time.
Let the batter rest for 24 hours in the fridge.
To cook the Yorkshire puddings preheat the oven to 220C/450F/Gas 7.
In a Yorkshire pudding tray with 8-12 holes place a little of the dripping in each hole and place in the hot oven until the fat is hot.
Remove from the oven and pour in the batter. Place in the oven for 20-25 minutes. Turn down the heat to 190C/375F/Gas 5 and cook for a further 10 minutes.
For the French onion soup, heat a large saucepan and add the butter and oil. Once hot, add the onions and thyme and fry for 20-25 minutes over a low heat, or until soft and golden-brown.
Add the garlic and cook for another couple of minutes then add the sherry and white wine. Cook until the liquid has reduced in volume by half.
Stir in the flour and cook for a further couple of minutes, then pour in the stock.
Bring to a simmer and cook gently for 10-15 minutes and season with the sugar, salt and pepper.
To make baguette croutons, preheat the grill to high.
Place the bread on a baking sheet and lightly toast under the grill, then add the grated cheese and return to the grill until golden-brown.
To serve, ladle some of the soup into warm bowls and top with the cheesy croutons.
Place the Yorkshire puddings a plate and serve the rest of the soup with them.