Cook the pasta: Bring a large pot of salted water to a boil. Add pasta and cook until just shy of al dente.
Reserve 1 cup of the pasta water, then drain.
Sear the mushrooms: Heat olive oil and butter in a large skillet over medium-high. Add mushrooms in an even layer and let them brown undisturbed for 3–4 minutes. Stir and cook until deeply golden and reduced, about 6–8 minutes total.
Add garlic: Reduce heat to medium.
Stir in garlic and cook 30–60 seconds until fragrant. Don’t let it brown.
Deglaze: Pour in the wine or broth. Scrape up the browned bits from the pan.
Simmer 1–2 minutes until reduced by about half.
Make it creamy: Lower heat to medium-low. Stir in the cream, salt, pepper, and red pepper flakes if using. Simmer gently 2–3 minutes to thicken slightly.
Add Parmesan: Sprinkle in Parmesan a handful at a time, stirring until smooth.
If the sauce feels too thick, add splashes of reserved pasta water to loosen.
Toss with pasta: Add the drained pasta to the skillet. Toss to coat, adding more pasta water as needed for a glossy, silky sauce that clings to the noodles.
Finish: Stir in lemon zest (if using) for brightness. Taste and adjust salt and pepper.
Top with parsley and extra Parmesan.
Serve hot: Plate immediately while the sauce is at its creamiest. Add a crack of black pepper on top.