Prepare the topping ahead. Gently saute the shallot and garlic in 1 tbsp oil for about 5 minutes until softened.
Remove from the pan into a bowl with a slotted spoon and set aside. Add a further 2 tbsp oil to the pan and saute the mushrooms over a high heat, stirring frequently for about 7 minutes until browned and softened. Add the wine and cook until it has reduced away. The mixture should be quite dry.
Add the mushrooms to the bowl with the onions and mix in the herbs. Season and allow to cool.
Whip the cream until softly stiff, then fold into the cooled mushroom mixture along with the egg yolk. Cover and chill until required.
Brush the steaks with the remaining oil and season them. Heat a large non-stick frying pan until it is really hot. Cook the steaks for about 2-3 minutes, turning them to sear all over. Remover from the pan.
Heat the oven to 220C. Put the steaks on ashallow baking tray. Pile the mushroom mixture on top of the steaks and dust with Parmesan. Cook, uncovered, for about 7 minutes or until the topping is bubbling and golden. serve immediately. Good with roast potatoes and a green vegetable.