Heat the oil in a pan. Sauté the onion, garlic and chilli pepper for approx. 3 mins. Add the celeriac and mushrooms, continue cooking for approx. 10 mins. Add the tomatoes, lentils and oregano, pour in the water. Bring to the boil, reduce the heat, cover and simmer for approx. 10 mins. Remove the lid and simmer for approx. 10 mins. until cooked, season.
Heat the oil in a frying pan, fry the aubergine slices for approx. 5 mins. on each side, season with salt and set aside.
Prepare the mashed potatoes according to the packet instructions, stir in the butter, season and leave to cool.
Transfer the lentil ragout to the prepared dish, place the aubergines on top. Spoon the mashed potatoes on top of the aubergines, smooth down. Sprinkle the Sbrinz cheese on top.
Gratinate for approx. 15 mins. in the upper half of an oven preheated to 220°C.