Preheat the oven to 180°C, gas mark 4. Bring a pan of salted water to the boil. Add the potatoes and simmer for 5 minutes. Drain, toss gently and set aside for 5 minutes.
Meanwhile, place a roasting tray with 1½ tbsp oil in the oven for 5 minutes until really hot. Carefully put the potatoes in the tray, scatter with the thyme, season and turn to coat. Bake for 30-35 minutes, or until crisp and golden.
Set a frying pan over a medium-high heat. Brush the steaks with the remaining oil, season and cook for 6-8 minutes, turning every minute, until done to your liking. Transfer to a warm plate, cover with foil and rest for 10 minutes.
Return the pan to the heat. Fry the mushrooms and garlic for 2-3 minutes, until light golden. Pour in the stock, reduce the heat and simmer for 2-3 minutes, or until the stock has reduced by a quarter. Stir in the crème fraîche and gently simmer until the sauce is thick and creamy. Season.
Toss the chips with the truffle oil. Slice the steaks and serve with the chips and mushroom sauce.