Preheat the oven to 400°F and scrub the potatoes clean.
Bake the potatoes for about 45 minutes until tender, then let them cool slightly.
Cut each potato in half lengthwise and scoop out most of the flesh into a bowl, leaving a thin layer to keep the skins sturdy.
Sauté the mushrooms in olive oil until soft, then add the spinach and cook until wilted.
Mix the potato flesh with the mushroom-spinach mixture, add cheese, and season with salt and pepper.
Spoon the filling back into the potato skins, top with extra cheese, and bake for another 10 minutes until melted and golden.