Bring a medium pot of salted water to a boil. Wash and dry all produce. Trim and thinly slice scallion, separating whites from greens. Trim and slice mushrooms into 1/4-inch-thick pieces.

Heat a large drizzle of olive oil in a large pan over medium-high heat. Add mushrooms; season with salt and pepper. Cook, stirring occasionally, until browned and slightly crispy, 5-7 minutes. Turn off heat; transfer to a paper-towellined plate. Wash out pan.

Once water is boiling, add cavatappi to pot. Cook, stirring occasionally, until al dente, 9-11 minutes. Reserve 1 cup pasta cooking water, then drain.

Meanwhile, melt 2 TBSP plain butter (4 TBSP for 4 servings) in pan used for mushrooms over medium-high heat. Add scallion whites and cook until just softened, 1 minute. Add flour and cook, stirring, until lightly browned, 1-2 minutes. Whisk in milk and 1/3 cup pasta cooking water (1/2 cup for 4), making sure to eliminate any flour clumps. Let simmer until slightly thickened, 3-4 minutes.

Stir cream cheese into pan with sauce until melted. Stir in mushrooms, cavatappi, and garlic herb butter. Season with salt and pepper. TIP: If needed, stir in reserved cooking water a splash at a time until pasta is coated in a creamy sauce.

Divide pasta between bowls and sprinkle with Parmesan and scallion greens. Serve.