Make the garlic butter: In a bowl, add the butter, minced garlic, salt and finely chopped parsley. Mix well, then shape into a sausage on plastic wrap and roll up tightly. Put back into the fridge to firm up.
Make the potatoes: Heat a large saucepan of salted water. Chop the potatoes into halves or quarters to make wedges. Boil the potatoes for 5 minutes, then drain and leave to dry in a colander.
Preheat the oven to 500 degrees or the hottest temperature. Put the potatoes on an baking tray. Drizzle with vegetable oil, and sprinkle with sea salt flakes. Cook for 15 to 20 minutes, turning twice, until golden brown.
Cook the mushrooms: Slice the butter into 6 wedges. Put one on each of the mushrooms and add to the oven for the last 10 minutes of cooking.
Cook the steak: Preheat a skillet or grill pan. Rub the steak with oil, salt and pepper. Cook for 2 1/2 minutes on each side, then transfer to a clean dish and cover tightly with foil. Rest for 5 minutes. Carve into slices and serve with the potatoes and mushrooms. Add slices of garlic butter on top of the steak.