Cook the Pasta:
Boil pasta in salted water according to package instructions. Reserve ¼ cup of pasta water, then drain.
Brown the Butter:
In a large skillet, melt butter over medium heat. Cook until it foams and turns golden brown with a nutty aroma—about 4–5 minutes. Stir constantly to avoid burning.
Sauté the Mushrooms:
Add sliced mushrooms to the brown butter. Cook over medium-high heat for 7–8 minutes until browned and tender. Season with salt, pepper, thyme, and red pepper flakes if using. Add garlic and cook for another 1 minute.
Toss the Pasta:
Add cooked pasta to the skillet along with Parmesan and reserved pasta water. Toss well until everything is coated and glossy.
Serve:
Plate the pasta and garnish with fresh parsley and extra Parmesan.