Peel and wash the potatoes and cut in half lengthways. Brush an ovenproof dish with a little olive oil. Place the potatoes side by side in the pan with the cut surface facing up, brush with a little oil and sprinkle with salt and pepper. Bake the potatoes in the preheated oven at 200 ° C (gas mark 3) for about 35 minutes until they are almost soft.
In the meantime, rinse off the parsley, shake it dry and pluck the leaves off. Peel the garlic and chop very finely with the parsley and the remaining oil in a blender. Finely chop the walnuts and mix with the parsley oil. Season to taste with salt, pepper and chili powder.
Drain the mozzarella and cut into slices. Spread the parsley pesto on the potatoes. Place mozzarella slices on top and bake the potatoes for another 10-15 minutes, until they are soft and the cheese layer is lightly browned.