Boil the pasta: Bring a large pot of salted water to a boil. Cook pasta until just shy of al dente. Reserve 1 cup of pasta water, then drain.

Sear the mushrooms: In a large skillet, heat olive oil and 1 tbsp butter over medium-high. Add mushrooms in an even layer with a pinch of salt. Cook undisturbed for 2–3 minutes, then stir and cook 3–4 minutes more until browned and tender. Transfer to a plate.

Sauté onions and garlic: Reduce heat to medium. Add remaining 1 tbsp butter, then onions and a pinch of salt. Cook 4–5 minutes until soft and lightly golden. Stir in garlic and red pepper flakes; cook 30 seconds until fragrant.

Deglaze and simmer: Pour in vegetable broth, scraping up any browned bits. Simmer 1–2 minutes to reduce slightly.

Make the light cream sauce: Lower heat to medium-low. Stir in half-and-half. Warm gently until steaming but not boiling. Add Parmesan in small handfuls, stirring until melted and smooth. If thick, loosen with a splash of pasta water.

Add veggies back: Return mushrooms to the pan. Add spinach and toss until just wilted, 1–2 minutes.

Toss with pasta: Add drained pasta to the skillet. Toss, adding reserved pasta water as needed to create a glossy, silky sauce that coats the pasta.

Finish and season: Stir in lemon juice. Taste and adjust with salt and pepper. Serve hot, topped with more Parmesan and herbs if you like.