Step  1

Cut the beef tenderloin into cross‑wise slices about 1 cm thick, then cut those slices into thin strips (matchsticks). Peel the onion and slice it either into feather‑like strips or half‑rings—whichever is easier for you.

Step  2

In a deep skillet, pour half of the vegetable oil (≈30 ml) and add the sliced onion. Cook over medium heat, adding butter during the process. Fry the onion until it becomes translucent and develops a light golden‑brown crust, about 5 minutes.

Step  3

Transfer the onion from the skillet to a bowl using a slotted spoon or perforated ladle, leaving most of the oil in the pan. Tilt the bowl so the onion gathers toward the top, allowing the oil that drains from the onions to collect at the bottom of the bowl.

Step  4

While the onion is frying, prepare the mushrooms. Wipe them with a damp cloth to remove any dirt, then slice the mushrooms thinly. Add them to the skillet that held the onion, along with the oil that drained into the bowl. Sauté the mushrooms over medium heat for 4–5 minutes, without adding salt. Do not evaporate all the moisture; the mushrooms should not become browned.

Step  5

At the same time, heat a second skillet over high heat, add the remaining vegetable oil (≈30 ml), and place the beef strips in it. Do not salt the meat. Fry over sufficiently high heat so the meat sears rather than stews. Stir regularly to keep the pieces separate. Cook for about 5 minutes.

Step  6

Add the sautéed onion to the skillet with the meat and stir. Season moderately with salt and ground black pepper, and fry for a couple of minutes while stirring. Then add the mushroom mixture together with its oil, and cook another 3 minutes. Transfer the entire sautéed mixture into a thick‑walled baking dish. Spread it out, but do not press it down—the interior should remain airy.

Step  7

Place a deep pot with frying oil on the stove and prepare the potatoes. Take young, well‑washed potatoes (no need to peel), slice them thinly into round chips, and deep‑fry them until cooked through. Drain the fried potatoes on paper towels.

Step  8

While the potatoes are frying, make the sauce. Combine egg yolks with heavy cream and soft “Fetax” cheese (a creamy cheese). Blend with a hand blender until smooth; the texture should be similar to sour cream.

Step  9

Layer the fried potato slices over the beef‑and‑mushroom mixture in the baking dish. Spread the creamy cheese sauce on top. Place the dish in the upper part of the oven preheated to 230 °C (450 °F) and bake for 10 minutes using the grill/broil setting.

Step 10

Meanwhile, finely chop fresh parsley. Dice the pickles into small cubes and drain any liquid. Remove the baked “beef stroganoff‑style casserole” from the oven. Sprinkle the diced pickles and chopped parsley over the browned surface. Serve hot.