For the crispy potato roulade with spinach leaves, preheat oven to 240°C top/bottom heat or 210°C hot air.
Peel the potatoes and cut them into very fine leaves (max. 2 mm thick). Line a baking tray with baking paper, spread half of the grated Parmesan evenly thinly on it. Place the potato slices on top in a fan shape, creating a scale pattern. Sprinkle the remaining Parmesan on top and pre-bake in the preheated oven for about 25-30 minutes until crispy.
Meanwhile, finely dice the onions and garlic. Wash the chard and also cut into fine strips.
Melt butter in a pan, sauté onions and garlic in it until colorless and add the chard. Stir-fry for about 3 minutes until about half of the volume has evaporated.
Add the spinach leaves, well squeezed out, and stir-fry for another 3-4 minutes over medium heat. Set aside, stir in the mascarpone and season with salt, pepper and nutmeg.
Allow the pre-baked potatoes to cool slightly. Spread the spinach-mangold mixture evenly in a thin layer on top. Then crumble the feta into pieces and sprinkle on top.
With the help of the baking paper, roll up broadside into a tight roll. Place the end of the roll in the middle of the baking paper and bake in the oven for 25 minutes at a constant temperature.
Cool the crispy potato roulade with spinach leaves for about 5 minutes before cutting.