Scrub and pat dry the potatoes, then pierce them with a fork.
Place the potatoes on a baking sheet. Season with salt.
Bake for about 1 hour, until the potatoes are cooked through. (test with a skewer)
Let the potatoes cool, then cut them half (lengthwise). Using a spoon, scoop out the potato flesh, making sure the skin doesn’t tear by holding the potato in your palm.
Place the potato skins, skin side down, on an oiled baking sheet. Bake for another 8-10 minutes until crispy and golden.
Meanwhile, remove the skin of the chorizo and finely chop the onion. Put them in a frying pan and gently cook, over low heat, until the chorizo releases its fat and the onion has softened.
Add the scooped potato flesh and combine, gently mashing into a coarse textured mixture.
Cut the olives into rings and fold into the mash. Season to taste.
When the potato skins are crispy, stuff them with the chorizo-potato mash. Season with ground pepper and scatter some grated cheese on top, if liked.
Bake for another 10 minutes or so until golden.
Serve warm.