Heat butter in a large pot over medium heat, then add mushrooms and onion. Cook until mushrooms are golden brown, about 5–7 minutes.

Stir in minced garlic and red pepper flakes (if using), cooking briefly until fragrant, about 30 seconds to 1 minute.

Pour in vegetable broth and bring to a gentle simmer, about 2–3 minutes. Scrape up any browned bits from the pot’s bottom to deepen flavor.

Add pasta to the broth, ensuring it’s submerged. Simmer uncovered, stirring occasionally, until pasta is al dente, about 10–12 minutes.

Lower heat and stir in heavy cream and grated Parmesan cheese, whisking until the sauce thickens into a silky coat, about 2–3 minutes.

Fold in fresh spinach leaves until gently wilted and vibrant green, about 1–2 minutes. Taste and season with salt and pepper.