Cook the beef tongue with the diced soup greens and salt in a pressure cooker with water that barely covers it for 60 minutes. Remove the tongue and peel off the skin while it is still warm.
Sauté the mushroom slices with 40g fat and chopped onions on medium heat for 7 minutes. Add the parsley, sour cream, and 40g grated cheese. Season with salt and pepper. Stir in breadcrumbs until the filling is no longer wet.
Melt 30g fat on medium heat, add the flour, and cook for 2 minutes, stirring constantly. Add the broth while stirring and bring to a boil. Reduce heat to medium-low, stir in the cream and remaining cheese, then remove from heat and mix in the egg yolk.
Cut the beef tongue into roughly 1cm thick slices. Layer the slices in a baking dish, placing a large tablespoon of filling between each slice. Pour the cheese sauce over the tongue and bake at 200°C (392°F) with both top and bottom heat for 20 to 30 minutes.