Cook the pasta according to the package.
While it’s cooking, heat up the butter over medium high heat in a large skillet. Add the garlic and mushrooms and cook, until there is some color on the mushrooms, about 2-3 minutes. Sprinkle on flour and stir to incorporate. Slowly stream in milk while whisking. Bring to a simmer and stir, until the sauce thickens slightly.
When the sauce is simmering, the pasta should be done cooking. Drain, reserving some pasta water. Add the pasta to the sauce and add the fresh spinach, tossing, to wilt. If needed, loosen up the sauce with some pasta water. Season with salt and pepper and enjoy!
