Bring a medium pot of salted water to a boil. Wash and dry produce. • Trim and slice mushrooms into ¼-inch-thick pieces. Trim and thinly slice scallions, separating whites from greens.
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-towel-lined 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.
While pasta cooks, 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 constantly, until lightly browned, 1-2 minutes. • Whisk in milk and 1⁄3 cup reserved pasta cooking water (½ cup for 4), breaking up any flour clumps. Simmer until slightly thickened, 3-4 minutes.
Stir cream cheese into pan with sauce until melted. • Stir in mushrooms, drained cavatappi, and garlic herb butter. Season with salt and pepper. TIP: If needed, stir in more reserved pasta cooking water a splash at a time until everything is coated in a creamy sauce.
Divide pasta between bowls; sprinkle with Parmesan and scallion greens. Serve.