Beef Fillet with Duxelle Mushroom Gratin

2 shallots, finely chopped
2 garlic cloves, crushed
4 tbsp olive oil
200g chestnut mushrooms, roughly chopped
1dl sherry wine
2 tbsp fresh thyme, finely chopped
½ dl double cream
1 large egg yolk
handful of bread crumbs
zest of 1 lemon
4 fillet steaks, about 180g and 4cm thick
4 tbsp freshly grated Parmesan

To prepare the topping, gently sauté the shallots and garlic in 1 tablespoon of olive oil for about 5 minutes until nicely softened. Add a further 2 tablespoons of olive oil and sauté the mushrooms over a high heat, stirring frequently, add the cherry and cook for about 5 minutes. The mixture should be quite dry. If necessary, tip it into a sieve to drain off any remaining liquid. Transfer the mushrooms to a bowl, allow to cool.

Whip the cream until softly stiff. Fold in the mushrooms along with the egg yolk, herbs, lemon zest, bread crumbs and a tablespoon of the grated Parmesan, then season with salt and pepper. Set aside.

Season the steaks all over with salt and pepper. Heat a large non-stick frying pan until you can feel a strong heat rising. Cook the steaks for about 2-3 minutes, turning them to seal all over. Remove from the pan. Put the steaks on a shallow baking tray. Pile the mushroom mixture on top of the steaks and dust with the remaining parmesan. Cook in the oven for about 5-7 minutes until the topping is bubbling and golden