Cook the pasta in a large pot of generously salted boiling water according to package directions until al dente. Just before draining, reserve about 1 cup of the starchy pasta water, then drain the pasta.

While the pasta is cooking, heat the olive oil and butter in a large skillet or pan over medium-high heat. Add the sliced mushrooms in a single layer and cook, without stirring for the first 2-3 minutes, until they are browned and caramelized. Continue to cook, stirring occasionally, for another 3-4 minutes.

Add the finely chopped shallot to the skillet and sauté for 2 minutes until it softens. Stir in the minced garlic and optional red pepper flakes and cook for about 1 minute more until fragrant, being careful not to burn the garlic.

Pour in the white wine to deglaze the pan, using a wooden spoon to scrape up any delicious browned bits from the bottom. Let the wine simmer and reduce by about half, which should take 1-2 minutes.

Reduce the heat to medium-low and pour in the heavy cream. Bring to a gentle simmer and let it cook for 2-3 minutes, allowing the sauce to thicken slightly.

Remove the skillet from the heat and stir in the grated Parmesan cheese until it's fully melted and the sauce is smooth. Add the fresh spinach in handfuls, stirring continuously until it has completely wilted into the sauce. Season with salt and pepper to taste.

Add the drained pasta directly to the skillet with the sauce. Toss everything together until the pasta is evenly coated. If the sauce seems too thick, add a splash of the reserved pasta water until it reaches your desired consistency.

Serve immediately, garnished with extra Parmesan cheese and fresh parsley if desired.