Peel the potatoes, cut them into quarters, boil and mash them well. Fry the resulting paste in 4 tablespoons oil, dilute it with milk, salt, sprinkle with black pepper and let it boil until it thickens. Remove the puree and let it cool slightly.
While stirring continuously, add three egg yolks. Pour the puree into a greased oven dish and put it to bake, sprinkle it with a little cheese and distribute small cubes of butter.
Sauté the mushrooms until ready, while also topping up from time to time with a little water. Once ready, remove from the heat, mix them immediately with the parsley and freshly squeezed lemon juice.
Scoop spoonfuls of the ready puree and serve these potato balls with your steaks, along with the mushrooms with spices.