Beef with pistachio

for the meatballs: one large onion , chopped and finely pureed 1 c finely chopped pistachios 1/4 c bread crumbs 1/2 c chopped tarragan 1/4 c chopped rosemary[...]

4 x Fillets steak, each weighting about 140g     Salt and freshly grnd black pepper 3 Tbsp. Vegetable oil 4 lrg Flat mushrooms 20 gm Unsalted butter 4 Tbsp.[...]