Bring the milk to the boil, remove from the heat and stir in the mashed potato powder. Stir in frozen herbs. Clean and wash the zucchini and aubergine and slice lengthwise into thin slices.
Place a few slices of courgette and aubergine next to each other on the work surface so that they overlap at the edges. Sprinkle with salt and pepper. Put some mashed potatoes in the middle and roll up.
Pin it with wooden skewers. Fry in hot oil at medium heat for 10-15 minutes. In the meantime, peel and finely dice the onion. Remove the rolls from the pan. Sauté onion in the frying fat.
Chop the tomatoes in the tin and add to the onion. Let it boil down in an open pot for about 10 minutes. Season with salt, pepper and oregano. Drain mozzarella and cut into slices. Pour the sauce into a round pie or tart dish.
Arrange the rolls on top. Cover with mozzarella slices. Sprinkle with pine nuts and bake in a preheated oven (electric: 200 °C/ gas: level 3) for approx. 10 minutes.