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 ½ 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 gently, for 3 to 4 minutes until the butter turns a warm golden brown 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 bits from the bottom of the pan. Let the sauce simmer gently for 2 to 3 minutes until it begins to thicken and coat 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 desired. Return the sautéed mushrooms to the pan and toss to combine.
07 - Add the drained pasta to the skillet and toss gently to coat every strand in the sauce. Gradually add the reserved pasta water, a splash at a time, until the sauce reaches a silky, clinging consistency.
08 - Taste and adjust the seasoning as needed. Remove from the heat, sprinkle with chopped fresh herbs, and serve immediately with extra Parmesan on the side.