Boil a large pot of salted water. Cook pasta until just al dente, then drain.
Heat olive oil and butter in a wide skillet over medium-high heat.
Sauté mushrooms until browned and their liquid cooks off, about 5–6 minutes.
Stir in onion and cook until softened and translucent, about 3 minutes.
Add garlic, Italian seasoning, and red pepper flakes. Cook 30 seconds until fragrant.
Pour in broth and scrape the pan. Simmer 2–3 minutes to reduce slightly.
Stir in cream, then add the cooked pasta. Toss to coat evenly.
Fold in spinach and cook until just wilted, 1–2 minutes.
Remove from heat. Mix in Parmesan and lemon juice. Season with salt and pepper. Garnish with parsley.