Combine milk, sunflower seed oil, egg yolk, flour, and 1/2 teaspoon salt in a bowl to form a smooth dough. Shape into a log, wrap in plastic wrap, and set aside.

Soak dried porcini mushrooms in 1/2 cup hot water for 45 minutes. Cook potatoes in boiling salted water for 20 minutes until tender.

Chop the rehydrated mushrooms and reserve the soaking liquid. Mash potatoes and mix with the mushroom-soaking liquid.

Heat vegetable oil in a skillet over medium heat. Cook and stir chopped onions until softened and translucent, about 5 minutes. Stir in chopped mushrooms.

Combine mashed potatoes, onions, and mushrooms. Season with salt and pepper to taste. Set filling aside.

Cut dough into discs about 3/8 inch thick and 2 inches wide. Roll each into thin circles. Place filling in the center, fold in half, and pinch the edges to seal.