Remove the tray from the oven, brush the potatoes with the butter (gently open the cuts with a knife, just to help get all that buttery goodness inside), and return them to the oven for a further 15 minutes, until golden and cooked all the way through.
Once the potatoes are ready, remove them from the oven (but leave the oven on), cut them in half and scoop out the flesh, so you are left with an even-ish layer of potato and skin (you can make this as thick or as thin as you like).
Heavily sprinkle the potatoes with salt and pepper, then place the baking tray on the middle shelf and bake the potatoes for 30–40 minutes.
Add the potatoes and onion to the pan and fry for 10 minutes, until there is a little colour on them.
One potato at a time, lay the handles of two wooden spoons either side of the potato, then make vertical cuts all along the length so that you cut almost through, but the spoon handles stop you going through completely.
