Peel the potatoes and cut them into thin slices. Peel the garlic, too, and cut into thin slices. Fry the mushrooms in a frying pan. Grease a large ceramic gratin pan with oil, sprinkle with grated breadcrumbs, thyme and put the garlic pieces.
Mix 2 teaspoons of salt, a third of a teaspoon of pepper and a third of a teaspoon of nutmeg.
Cover the bottom of the mold with a layer of potatoes, which sprinkle with a mixture of salt and spices.
Mix sour cream and 1 cup of milk in a bowl. Pour two tablespoons of this mixture over the potatoes and sprinkle with one tablespoon of grated cheese.
Again lay out a layer of potatoes, sprinkle it with salt and spices again, pour the sour cream mixture (2 tablespoons), lay out a layer of mushrooms (they are only 1 layer and enough).
Then lay out the potatoes again, sprinkle-pour, pour cheese. And another layer of potatoes. We sprinkle it with salt and spices again, pour the remnants of the sour cream mixture, pour 1 cup of kefir into the mold, sprinkle with a pinch of thyme and put it in a preheated oven to 200 degrees for 45-60 minutes.
Then remove the mold from the oven and sprinkle the browned potatoes over the remains of grated cheese (there should be a decent amount of them).
Put it back in the oven and bake for 15-20 minutes until an appetizing golden crust forms.