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. Reserve 1/2 cup of the starchy pasta water before draining.
02 - While the pasta cooks, heat olive oil and 2 tbsp butter in a large skillet over medium-high heat. Add the sliced mushrooms in an even layer and sauté without stirring for the first 3 minutes to develop color. Continue cooking 7–8 minutes total until the mushrooms are deeply golden and all released moisture has evaporated. Transfer 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, the milk solids turn golden brown, and a fragrant nutty aroma develops. Watch carefully to prevent burning.
04 - Add the minced garlic to the browned butter and sauté for 1 minute, stirring constantly, until softened and fragrant.
05 - Pour in the heavy cream and whisk to combine, scraping up any caramelized browned 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, 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 is too thick, add the reserved pasta water a splash at a time until the desired consistency is reached.
08 - Taste and adjust the seasoning with additional salt or pepper as needed. Remove from the heat, sprinkle generously with chopped parsley or chives, and serve immediately with extra Parmesan on the side.