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 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 undisturbed for 3 minutes, then stir occasionally for another 4-5 minutes 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 tbsp butter to the same skillet. Cook, swirling the pan occasionally, for 3-4 minutes until the butter foams, then turns a warm 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 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 pan and toss to coat in the sauce.
07 - Add the drained pasta to the skillet and gently toss until every strand is evenly coated in the sauce. If the sauce feels too thick, add the reserved pasta water a splash at a time until it reaches a silky, velvety consistency.
08 - Taste and adjust the seasoning with additional salt or pepper as needed. Remove from the heat, sprinkle generously with the chopped fresh herbs, and serve immediately with extra Parmesan on the side.