For the oyster mushroom casserole with chive sauce, boil the potatoes, let cool, peel and coarsely grate. Remove the seeds from the zucchini and cut into slices.
Clean and chop the oyster mushrooms. Peel carrots and cut into thin slices. Peel tomatoes and cut into cubes.
Cut an onion into rings, press the garlic, fry both in a little oil. Gradually add zucchini, oyster mushrooms, carrots and tomatoes. Stir well, remove from heat and season with salt and pepper.
Cut the second onion into small cubes and fry in a pan in oil until translucent. Add the minced meat, mix well and fry over low heat for about 10 minutes, stirring frequently. Season with salt and pepper.
Mix sour cream with eggs and grated Parmesan, season with salt, pepper and nutmeg and add to the grated potatoes.
In a greased baking dish, first pour the mashed potatoes, then the roasted mince and finally the roasted vegetables. Coarsely grate the mozzarella and sprinkle on top.
Bake in the preheated oven on the middle shelf at 200°C for about 20 minutes.
For the chive sauce, mix sour cream, cream yogurt and curd cheese. Season with salt and pepper. Cut the chives into rolls and mix in.
Arrange the oyster mushroom casserole on plates and serve with a few spoonfuls of chive sauce per serving.