In a large pot of salted water, cook pasta until al dente. Reserve 1/2 cup of pasta water. Drain pasta and set aside.

Meanwhile, in a large skillet, melt butter over medium/high heat. Cook, stirring until the butter starts to turn golden, about 3 minutes.

Add mushrooms and cook, stirring occasionally until golden in places, about 8 minutes. Add garlic and cook, stirring until garlic is fragrant, about 1 minute.

Add vinegar and red pepper flakes to skillet with mushrooms, season with salt and pepper, cook over medium heat stirring until vinegar is evaporated, about 2 minutes.

Pour in heavy cream and Parmesan, stir to combine. Add spinach and cook until spinach starts to wilt.

Add cooked pasta and reserved pasta to skillet. Cook, tossing until everything is coated in sauce, about 2 minutes. (if you would like a little more sauce or for it to be saucier just add more heavy cream)

Remove from heat, garnish with more Parmesan and red pepper flakes!