For the onion gravy:
1. Pre-heat the grill to high.
2. Heat a large sauté pan and add the butter and oil. When hot add the onions and thyme and fry for 20-25 minutes over a low heat, until soft and golden brown.
3. Add the garlic and cook for another couple of minutes, then add the sherry and white wine and cook until the liquid has reduced by half.
4. Stir in the flour and cook for a further minute, then pour in the stock.
5. Bring to a simmer and cook gently for 10-15 minutes, and season with the sugar, salt and pepper.
6. Place the bread on a baking sheet and lightly toast, then add the grated cheese and return to the grill until golden brown.
For the Yorkshire puddings:
1. Pre-heat the oven to 220°C, gas mark 7.
2. Place the flour in a large glass bowl and season with salt and pepper. Add the eggs and whisk and gradually add the milk.
3. Let the batter rest for 24 hours.
4. Place a little of the dripping in a Yorkshire pudding tray and place in the oven until the fat is hot.
5. Remove from the oven and pour in the batter. Place in the oven for 20-25 minutes. Turn down the heat to 190°C, gas mark 5 and cook for a further 10 minutes.