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 cook undisturbed for 3 minutes, then stir and continue sautéing for another 4-5 minutes until deeply golden and all released moisture has evaporated. Transfer 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 nutty, toasted 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 lightly golden.
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 2-3 minutes until it coats the back of a spoon and slightly thickens.
06 - Stir in the grated parmesan until melted and smooth. Season with salt, freshly ground 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 sauce. If the sauce is 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 as needed. Remove from heat, garnish with chopped fresh parsley or chives, and serve immediately with extra parmesan on the side.