Bring a large pot of salted water to a boil. Cook the pasta until al dente, reserving ½ cup of the cooking water, then drain.

Meanwhile, heat olive oil and butter in a large skillet over medium heat. Sauté the onion and garlic until soft and fragrant. Add the portobello mushrooms and cook until golden and meaty.

Add the cream, spinach (if using), and the mushroom soaking liquid, including the chopped shiitake. Bring to a gentle simmer and season with salt and black pepper.

Toss the cooked pasta into the sauce to coat. If needed, loosen with the reserved pasta water. Stir in Parmesan and butter until the sauce thickens and clings to the pasta.

Serve immediately, topped with extra Parmesan and a sprinkle of parsley, if desired.