Dice the leek and finely chop the thyme. In a small saucepan melt half the quantity of butter. Add diced leek and thyme and cover with lid. Cook until soft and the leek has become semi-transparent. Set aside.
Bring a pot of water to boil. Whilst water is boiling, slice all mushrooms. In a warm pan, add the extra virgin olive oil and mushrooms. Cook on a medium heat until golden in colour. Add the white wine and cook until evaporated. Add garlic, braised leek, chicken stock and remaining butter, and simmer until the sauce has thickened. Season to taste. Once water has boiled, add salt and pappardelle pasta and cook until al dente.
In the pan with mushrooms, add truffle oil, dry porcini mushroom dust and the pappardelle pasta. Toss and serve with shaved pecorino.