Bring a large pot of salted water to a boil and cook the pasta according to package instructions but reduce cooking time by 2 minutes so pasta is al dente; drain well and save 1 cup of pasta water.
Heat a large skillet over medium heat, add 2 tablespoons olive oil, then add the mushrooms and black pepper, and cook until golden brown and softened, about 5-10 minutes. Add the garlic and cook with the mushrooms until fragrant, about 1-2 minutes. Set aside.
In the same skillet used to cook the mushrooms, add the remaining olive oil over medium heat, add the flour and stir to combine. Slowly pour the milk on top, whisking to combine. Lower the heat and simmer until the mixture thickens.
Add the cooked pasta on top, along with the mushrooms, fresh parsley and as much of the pasta water as you’d like. Toss to combine.
Serve immediately with parmesan cheese, if desired.