Preheat the oven to 180°C. Put the mushrooms in the oven bag, stem side up, and into a large ovenproof pan. To the bag, add the stock and soy sauce and massage gently using your hands. Bake for 20-25 minutes, until cooked through. Remove the mushrooms from the oven and remove from the oven bag. Transfer the mushrooms to sieve to drain. Discard the liquid and pat dry each mushroom very well.

Meanwhile, make the sauce. In a bowl, combine all the sauce ingredients and mix well. When the mushrooms are cool, place two of them on a plate (stem side up) and divide the rest of the sauce between them. Sandwich them with the other two mushrooms (stem side down) and press them down to seal in the butter. Place the flour, beaten eggs and panko in three separate bowls. Season the panko with salt. Coat each mushroom sandwich in the flour, shaking off any excess, then dip into the egg mixture followed by the panko. Transfer to a plate and freeze for 1 hour. Heat the oil in a medium, high-sided saute pan over medium heat. When the oil is warm enough, add the mushrooms and fry for about 4 minutes on each side, until deeply golden. Transfer onto paper towels.Divide yoghurt between two plates and serve the warm mushroom kyivs on top.