Pre-heat the oven to 400°F.
Rub the potatoes with oil and season with salt and pepper.
Cut the end of the garlic off and drizzle with oil; season with salt and pepper. Wrap the garlic in foil with the rosemary sprig. Transfer the potatoes and garlic into the oven, roasting the potatoes for 1 hour, removing the garlic after 45 minutes and cooling to handle.
Halve the potatoes and scoop flesh into a bowl. Arrange the potato skins on a baking sheet. Squish the garlic cloves from their skins. Mash the potatoes with milk, salt, pepper, nutmeg, garlic and Gruyère. Fill the potato skins and top with ratatouille. Sprinkle with Parm and return to the oven for a few minutes to brown the cheese a bit.
Serve 2 stuffed skins per person.