Put the flour, butter, milk, beer, cheese and mustard into a pan, set over a medium heat and stir with a whisk until thickened and smooth. Add the Worcestershire sauce and cayenne. Cover the surface with a piece of baking parchment or cling film to prevent a skin forming and set aside to cool. You can make the rarebit sauce up to three days before using (store in the fridge).

Heat the oil in a frying pan. Cook the onions in the pan for 15-20 mins until caramelised. Add the sugar and Marmite to the onions and cook for another 2-3 mins until sticky, adding a splash of water if it looks dry.

Toast the slices of bread lightly and heat the grill. If you’re serving with an egg, heat a frying pan with a drizzle of oil. Spread 2 slices of toast with the onions and top with half the rarebit mixture. Cover the remaining 2 slices of bread with the rest of the rarebit mixture. Place under the grill until bubbling (about 2-3 mins), then sandwich the slices together with the cheesy onions in the middle. While they’re cooking, fry the eggs to your liking. Top the sandwiches with a few shakes of Worcestershire sauce, a grinding of pepper and the eggs.