Preheat the oven to Gas Mark 4, 180C (350F).
To cook the potatoes: Put the potatoes, milk, cream, garlic, nutmeg and a little salt and pepper in a large saucepan and bring to the boil. Reduce the heat and simmer for 3 minutes, stirring regularly.
Remove from the heat and transfer the potatoes and sauce to an ovenproof baking dish. Bake for 40-50 minutes until the potatoes are cooked and the top is golden.
To Cook the steaks: When you are ready to cook the steaks heat a pan or barbecue until really hot.
Brush a little olive oil over the steaks. Season both sides with a little salt and black pepper just before placing them on the hot pan or barbecue.
Cook for two minutes, then turn and cook for two minutes on the other side. Depending on how thick the steaks are and how you like them cooked, you may need to cook them for a further one or two minutes on each side. Let the steaks rest for a couple of minutes before serving.
To make the sauce: While the steaks are cooking combine all the ingredients for the Chimichurri Sauce, except the oil, in a mini processor. You can also chop everything finely by hand. Whizz for a couple of minutes then add the oil, season with a little salt and black pepper and whizz again.
Serve the steaks with the Chimichurri Sauce, sauteed mushrooms and garlic potatoes.