Wash and peel the potatoes, then slice them into even rounds or half-rounds about 0.3 cm thick.
Prepare the aromatics: peel and finely chop the onions and garlic; wash, drain and chop the dill.
Toss the potatoes, onions, garlic and dill in a large bowl with the black pepper and 1 tsp salt.
Stir in the milk kefir until all the vegetables are coated.
Lightly grease the base and sides of the baking dish with vegetable oil.
Pour the vegetables and all their liquid into the dish, spreading them evenly with a spoon.
Bake for about 40 minutes and test a potato slice in the centre with a knife. It is ready when the knife slides through easily; if the potato is still firm, keep baking and test again before serving.
Storage and safety
Refrigerate leftovers promptly in shallow containers within 2 hours, at 4°C (40°F) or below. Eat within 2–3 days. Reheat until the centre reaches 74°C (165°F).