Bring a medium pot of salted water to a boil. Wash and dry all produce. Trim, then thinly slice scallions, separating whites from greens. Thinly slice mushrooms (about 1/4-inch-thick).

Heat 1 TBSP 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. Rinse and wipe out pan.

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

Melt 2 TBSP plain butter in same pan used to cook mushrooms. Add scallion whites and cook until just softened, 1-2 minutes. Add flour. Cook, stirring often, until lightly browned, about 2 minutes. Whisk in milk and 1/3 cup pasta cooking water, making sure to eliminate any flour clumps. Let simmer until slightly thickened, 3-4 minutes.

Stir cream cheese into pan until melted. Stir in mushrooms, gemelli, and garlic herb butter. Season with salt and pepper. TIP: Everything should be coated in a loose, creamy sauce. If very thick, add more pasta cooking water 1 TBSP at a time.

Divide pasta between plates and sprinkle with Parmesan. Garnish with scallion greens and serve.