Turn the oven on to 200°. Wash the potatoes well, dry them and wrap them one by one in aluminium foil. Place them on a baking tray and bake for about 1 hour and 30 minutes.
While the potatoes are cooking, wash the spinach and dry well. Blend with the pine nuts and a pinch of salt, adding the oil until the pesto is smooth. Keep it aside.
Shortly before the end of the cooking time of the potatoes, remove the rind from the Fontina and grate it with a large-hole grater.
Check the cooking point of the potatoes by pricking them with a fork, remove them from the oven and, handling them carefully, remove the foil and cut them lengthwise.
Season each potato with salt, pepper and a spoonful of pesto. Stuff them with a quarter of the Fontina and 2 slices of Arnad Lard and serve immediately.