01 - Bring a large pot of generously salted water to a rolling boil. Add the fettuccine or linguine 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 the olive oil and 2 tablespoons of butter in a large skillet over medium-high heat. Add the sliced mushrooms in an even layer and sauté for 7 to 8 minutes, stirring occasionally, until deeply golden and all released moisture has evaporated. Transfer the mushrooms to a plate and set aside.
03 - Reduce the heat to medium. Add the remaining 5 tablespoons of butter to the same skillet. Cook, swirling the pan occasionally, for 3 to 4 minutes until the butter foams, turns a warm golden brown color, and releases a nutty aroma.
04 - Add the minced garlic to the browned butter and sauté for about 1 minute, stirring constantly, until fragrant and lightly softened.
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 to 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 pan and toss to coat in the sauce.
07 - Add the drained pasta to the skillet and toss gently to coat every strand in the velvety sauce. If the sauce feels too thick, loosen it by adding 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 heat, sprinkle with chopped fresh parsley or chives, and serve immediately with extra grated Parmesan on the side.