Boil the pasta: Bring a large pot of salted water to a boil and cook the pasta according to the package directions until al dente. Reserve about 1 cup of the pasta water, then drain and set the pasta aside.

Brown the mushrooms: Heat the butter and olive oil in a large skillet over medium-high heat. Add the mushrooms in as close to a single layer as you can and let them cook undisturbed for 3-4 minutes, then stir and cook 3-4 minutes more until deeply golden. Sprinkle with 1/4 teaspoon of the salt near the end.

Add the garlic: Stir in the garlic and Italian seasoning and cook for about 30 seconds, just until fragrant, so the garlic doesn't burn.

Make the cream sauce: Pour in the heavy cream and bring it to a gentle simmer, scraping up any browned bits from the bottom of the pan. Simmer for 2-3 minutes until slightly thickened, then reduce the heat to low and stir in the parmesan until smooth.

Wilt the spinach: Add the spinach a few handfuls at a time, stirring until it wilts down into the sauce, about 1-2 minutes. Season with the remaining 1/4 teaspoon salt, the pepper, and red pepper flakes if using.

Combine and serve: Add the drained pasta and toss to coat, loosening the sauce with a splash of the reserved pasta water if it's thicker than you'd like. Serve warm with extra parmesan on top.