Heat oven to 200C/fan 180C/gas 6. Roll out the pastry to about the thickness of a £1 coin, then cut out 2 x 12cm circles using a cutter, or by scoring around a saucer. Score a circle 2cm in from the edge, then prick the pastry inside the border. Lift onto a baking tray, then bake for 20-25 mins or until golden and puffed. Press the risen middles down a little, ready for the filling. Can be made a day ahead, then reheated in a hot oven.
Heat the oil in a pan, add the shallots, then fry until softened. Add the mushrooms and thyme, then fry until mushrooms are softened and any liquid almost gone. Add the port or Madeira, then bubble for 2 mins. Add the cream, simmer for 1 min more until the sauce is slightly thickened, then set aside.
Rub the steaks with a little oil and seasoning. Heat a griddle pan until hot, then cook the steaks for 2-3 mins on each side (depending on their thickness) for medium rare, a little more if you like your steaks well done. Cover the steaks with foil, then rest them for 10 mins.
Reheat the tartlets. Warm the mushroom mixture over a low heat. Set the tartlets on warm plates, then spoon over the mushroom mixture. Sit the fillet steaks on top with a sprig of thyme. Serve with Balsamic spinach and Oven sauté potatoes (see below).