Bring a large pot of water to a boil and add 1 tablespoon of salt. Cook the fettuccine according to package instructions until al dente. Before draining, scoop out at least 1 cup of pasta cooking water and set aside in a heatproof container.

While pasta cooks, heat a large frying pan over medium-high heat. Add the butter and olive oil. Once the butter sizzles, add the sliced mushrooms in a single layer. Cook undisturbed for 2 minutes, then stir and cook for another 1 to 2 minutes until deeply golden brown. Do not salt the mushrooms yet.

Add the crushed garlic and rosemary to the pan. Sauté for 60 seconds, stirring constantly, until fragrant.

Pour in the cream and lemon juice. Reduce heat to low and simmer for about 5 minutes, stirring frequently, until the sauce has thickened slightly.

Add the drained pasta directly to the pan and toss well to coat. If the sauce is too thick, add the reserved pasta water a few tablespoons at a time, tossing between additions, until the sauce reaches a smooth, glossy consistency that clings to the pasta.

Remove the pan from heat. Stir in the freshly grated Parmesan cheese. Season generously with salt and freshly ground black pepper.

Serve immediately, topped with an extra crack of black pepper and fresh parsley if available.