01 - Bring a large pot of generously salted water to a rolling boil. Add the fettuccine and cook according to package directions until al dente. Before draining, reserve 1/2 cup of the starchy pasta water. Drain the pasta and set aside.
02 - While the pasta cooks, heat the olive oil and 2 tbsp butter in a large skillet over medium-high heat. Add the sliced mushrooms in an even layer and sauté for 7–8 minutes, stirring occasionally, until they are deeply golden and all released moisture has cooked off. Transfer the mushrooms to a plate and set aside.
03 - Reduce the heat to medium. Add the remaining 5 tbsp butter to the same skillet. Cook, swirling the pan occasionally, for 3–4 minutes until the butter foam subsides, turns a warm golden-amber color, and releases a fragrant nutty aroma. Watch carefully to prevent burning.
04 - Add the minced garlic to the browned butter and sauté for about 1 minute, stirring constantly, until fragrant and softened.
05 - Pour in the heavy cream, whisking to incorporate and scraping up any caramelized brown bits from the bottom of the pan. Let the sauce simmer gently for 2–3 minutes until it slightly thickens and coats the back of a spoon.
06 - Stir in the grated Parmesan cheese until melted and smooth. Season with salt, freshly ground black pepper, and a pinch of nutmeg if using. Return the sautéed mushrooms to the skillet and toss to coat in the sauce.
07 - Add the drained pasta to the skillet and toss gently to coat every strand in the sauce. If the sauce feels too thick, add the reserved pasta water a splash at a time until the desired silky consistency is reached.
08 - Taste and adjust the seasoning with additional salt or pepper as needed. Remove from heat, sprinkle generously with chopped fresh parsley or chives, and serve immediately with extra Parmesan on the side.