Bring a medium pot of salted water to a boil. Wash and dry produce.

Trim and slice mushrooms into ¼-inch-thick pieces. (Skip if your mushrooms are pre-sliced!) Trim and thinly slice scallions, separating whites from greens.

Heat a large dry pan over medium-high heat. Add bacon*; cook, turning occasionally and adjusting heat if browning too quickly, until crispy, 6-10 minutes. Transfer bacon to a paper-towel-lined plate. Once bacon is cool enough to handle, roughly chop. Wipe out pan. 

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.

Use pan used for bacon. 

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, until lightly browned, 1-2 minutes.

Whisk in milk and ¼ 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 and combined.

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

Stir in bacon along with mushrooms.

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