To make the filling, slice the onions and then heat a large non-stick pan over medium-low heat.

Add the butter and allow it to brown. This will give the onions a slightly nutty flavour.

Add the onions along with the bay leaves and fresh thyme and then season with salt and pepper. Turn the heat down to low.

Once the onions have softened and started to take on some colour (this will take about 10-15 minutes), add the sherry if you are using it and put a lid on the pan. Continue to cook them for another 10 – 15 minutes. Stir occasionally. If necessary, add a splash of the onion stock to deglaze the pan and prevent sticking.

Once the onions are nice and golden, remove the lid, and add a splash more stock. Allow them to cook until very golden brown, and sticky. The process to caramelize onion takes at least half an hour, but this part can be made in advance.

Decant the onions to a bowl to cool and remove the bay leaves.

Once the onions have cooled slightly, add the grated cheese, and sherry (or red wine) vinegar and stir to combine.

Butter one side of each slice of bread. Dived the onion and cheese mixture into 2 and spread it thickly onto two of slices of sourdough. Ensure the buttered sides are on the outside.

If you are using them, press 2 sage leaves into one side of each sandwich (this is optional).

Fry the sandwiches in a large skillet over medium to low heat until golden and toasted and the filling is warm and melty. They should take around 3 – 4 minutes per side and keep a close eye to prevent the bread from going too dark before the cheese has melted.

Alternatively, toast the sandwiches on a sandwich press or in an air fryer too.

Once the sandwich is a deep golden brown colour, allow it to rest for a minute then slice and enjoy.