Heat oven to 200ºC / Fan 180ºC / Gas Mark 6. Roll out the pastry to about the thickness of a £1 coin, then cut out two equal squares. Score a 2cm in from the edge, then prick with a fork, the pastry inside the border. Lift onto a baking tray, then bake for 20-25 mins or until golden and puffed. Can be made a day ahead, then reheated in a hot oven.
Heat the oil in a pan, add the onion, then fry until softened. Add the mushrooms and thyme, then fry until mushrooms are softened and any liquid almost gone. Add the white wine, then bubble for 2 mins. Add the cream, simmer for 1 min more until the sauce is slightly thickened, then set aside.
Steam for 10 minutes the tenderstem broccoli. Rub the steaks with a little oil and seasoning. Heat a griddle pan until hot, then cook the steaks as per your own preferences.
Set the tartlets on warm plates, sit the fillet steaks on top with a sprig of thyme and then spoon over the mushroom mixture. Serve with cooked tenderstem broccoli.