Nutty Gruyere cheese and fresh green watercress really make this pasta special.
fresh cracked pepper
Clean the mushrooms and slice them. Set aside. Cook the fettuccine until just al dente. Melt 3 tablespoons of the butter in a large skillet and saute the onion until translucent. Add the mushrooms to the pan, salt them, and saute until they cook down and soften a bit. This will only take a few minutes, don't cook them into oblivion. Season with fresh cracked pepper and thyme. Add the sherry and chicken broth, if using, and let the liquid reduce for a minute or two on a medium high heat. Add the last tablespoon of butter to the sauce, check the seasonings, and keep warm. Drain the pasta and toss with the watercress and cheese. Add to the skillet and toss well, heating everything through. Scatter the hazelnuts liberally over the pasta.