Bring a large pot of salted water to a boil. Cook the penne pasta just shy of al dente, about one minute less than package suggests. Reserve 1/2 cup of pasta water before draining.
In a large skillet, warm olive oil over medium-high heat. Add sliced mushrooms and cook undisturbed for about 5 minutes until browned.
Add shallot, garlic, and red pepper flakes. Season with salt and black pepper, cooking until mushrooms are caramelized and fragrant.
Stir in butter and squeeze in lemon juice. Let the mixture bubble as the butter melts.
Add drained pasta to the skillet along with spinach and reserved pasta water. Simmer for a few minutes until spinach wilts and pasta is coated.
Fold in lemon zest and grated parmesan cheese until melted and well combined.
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Notes
Serve hot, garnished with extra parmesan or fresh herbs. Store leftovers in an airtight container for up to 3 days.