- Place the potatoes in a pot and cover with cold water. Season with salt. Bring to a boil and cook for 12 minutes or until tender. Drain and let cool.
- Meanwhile, in a large skillet over medium heat, cook the spinach in the oil until wilted. Drain in a sieve and press down to extract any excess liquid. Set aside.
- With the rack in the middle position, preheat the oven to 400°F (200°C). Line a baking sheet with a silicone mat or parchment paper.
- Cut the potatoes in half. Cut the rounded part off each potato half so they sit flat.
- With a melon baller, remove all but ¼ inch (5 mm) of the flesh from each potato half. Place the potato flesh in a bowl and mash with a fork. Place the potato halves on the baking sheet.
- On a work surface, grate 2 oz (55 g) of the cheese. Cut the remaining cheese into 24 very thin slices.
- To the bowl of mashed potatoes, add the spinach, sour cream, mustard and grated cheese. Season with salt and pepper. Fill the potato halves with the mixture.
- Bake for 15 minutes. Place one slice of cheese on top of each potato half. Bake for 1 more minute. Let sit for 5 minutes before serving.