Drain the porcini, reserving the soaking liquid, then squeeze dry and roughly chop. Heat 3 teaspoons of the olive oil in a small frying pan over a medium heat, add the garlic and fry for 30 seconds. Add the mushrooms and fry for 1–2 minutes, then stir in 3 tablespoons of the reserved soaking liquid, 3 tablespoons of the stock and 1 tablespoon of the chives. Bubble for 1 minute, then remove from the heat and keep warm.
Rub 1 teaspoon of the remaining olive oil over each of the steaks, season with pepper and place on a preheated hot griddle pan. Cook for 2–5 minutes on each side until cooked to the pinkness desired. Remove and leave to rest for 4–5 minutes.
Pour the remaining mushroom soaking liquid and stock into a large saucepan and bring to the boil. Pour in the polenta in a steady stream, whisking gently until the polenta thickens, then reduce the heat and cook for 2 minutes. Stir in the Parmesan, truffle oil and 4 tablespoons of the chives. Season to taste.
Spoon the polenta onto serving plates and spoon over the mushrooms and sauce. Slice the steaks in half diagonally and place on top with any cooking juices. Sprinkle with pepper and a scant pinch of truffle salt, if using. Garnish with the remaining chives and serve immediately with griddled courgettes (see below).