Bring a large pot of salted water to a boil. Add the fettuccine or spaghetti and cook according to package instructions until al dente. Drain and set aside, reserving about ½ cup of pasta water.
In a large skillet, heat the olive oil over medium heat. Add the chopped onion and sauté for about 3-4 minutes until translucent. Add minced garlic and cook for an additional minute until fragrant.
Add the sliced cremini mushrooms to the skillet. Cook for 5-7 minutes, stirring occasionally, until the mushrooms are browned and have released their moisture.
Stir in the chopped spinach and cook until wilted, about 2-3 minutes.
Pour in the vegetable broth and heavy cream, stirring to combine. Add the dried Italian herbs and season with salt and pepper. Let the sauce simmer for about 5 minutes, allowing it to thicken slightly.
Add the cooked pasta to the skillet, gently tossing it with the mushroom-spinach mixture. If the sauce is too thick, add some reserved pasta water to reach the desired consistency.
Once everything is well combined and heated through, remove from heat. Serve the pasta warm, garnishing with freshly grated Parmesan cheese and chopped parsley if desired.