Secure the spatchcocks with kitchen string. Place in a large baking dish with the garlic, lemon rind, lemon juice, thyme and oil and toss to coat. Refrigerate for at least 1 hour to marinate.Preheat oven to 200°C (400°F). Roast the spatchcocks for 35–40 minutes or until cooked through and golden.While the spatchcocks are roasting, make the porcini mash. Place the porcini in the bowl of a small food processor and process until finely chopped. Place the potatoes and porcini in a large saucepan of salted cold water. Bring to the boil and cook for 12–15 minutes or until potatoes are cooked through. Drain and return the potato and the porcini to the pan with the milk, butter, salt and pepper and mash until smooth. Set aside and keep warm.To make the gremolata, place the parsley, lemon zest, garlic, oil, lemon juice, salt and pepper in a bowl and mix to combine. Divide the spatchcocks and porcini mash between plates and top with the gremolata to serve.
+ You can buy dried porcini mushrooms at delicatessens and specialty food stores.