Heat the oven to 220C. Put the flour, baking powder, salt and mustard powder into a large mixing bowl and whisk together.
Grate in the butter, then rub it in with your fingertips until it resembles breadcrumbs.
Finely grate in 225g cheese, add the chives, and then stir to combine. Mix in the milk and water until the dough just comes away from the edge of the bowl; don’t handle it any more than is necessary. Tip on to a very lightly floured surface and flatten into a large circle.
Put on a baking tray and brush the egg and milk mixture. Grate the remaining cheese over the top and bake for about 40 - 50 minutes until golden. Allow to cool slightly on a rack before slicing.
To make the toasties, place the sliced bread on to a board and butter each side. Place a handful of cheese on top of one slice and sandwich together with a second. Spread the top of the sandwich with mayonnaise and sprinkle some more cheese on top. Heat up a toastie maker/panini press and place the sandwich, cheese side down on top of the press. Spread the top side with a little more mayo and sprinkle on some more cheese. Place a few sprigs of thyme on top and cook until golden and bubbling.
To make the gravy, heat the beef dripping in a saucepan and add the onions, frying until golden. Add the flour and cook out for 1 minute then add the mustard powder, beef stock and worcestershire sauce, stirring until thickened.
Serve the sandwich alongside the warm onion gravy for dipping.