Peel and wash the potatoes and slice them finely. Clean, wash and grate carrots and celery. Wash, clean and also grate the zucchini. Heat the ghee in a large pan, sauté the carrots, zucchini and celery for about 4 minutes, season with salt, pepper and turmeric.
Wash the herbs, shake dry and chop finely except for a little to garnish. Mix the herbs into the vegetables. Line a thin casserole dish (20 x 30 cm) with potatoes, season with salt and nutmeg.
Add a layer of vegetables and repeat the process twice.
Mix the egg yolk and 2 tablespoons of cream, spread the top layer of potatoes with it and bake in a preheated oven (electric cooker: 175 °C/ convection oven: 150 °C/ gas: see manufacturer) for approx. 40 minutes. Remove the casserole from the oven and sprinkle with the remaining herbs.