Preheat oven to 400 degrees. Bring a large pot of salted water to a boil. Wash and dry all produce. Trim and thinly slice scallions, separating whites from greens. Toss cauliflower on a baking sheet with a large drizzle of olive oil and a pinch of salt and pepper. Roast until tender, 20-25 minutes.

Once water is boiling, add cavatappi to pot. Cook, stirring occasionally, until al dente, 9-11 minutes. Reserve 1 cup pasta cooking water (11/2 cups for 4 servings), then drain.

Meanwhile, heat a large, dry pan over medium-high heat. Add bacon and cook, flipping occasionally, until just crispy, 6-10 minutes. Turn off heat. Transfer bacon to a paper-towel-lined plate. Discard all but 1 tsp bacon fat (2 tsp for 4 servings) from pan.

Melt 1 TBSP butter (2 TBSP for 4 servings) in same pan over medium heat. Add scallion whites and cook until softened, 1 minute. Add flour and stir constantly until lightly browned, 1-2 minutes. Slowly whisk in milk, 1/4 cup reserved pasta cooking water (1/2 cup for 4), and stock concentrate. Increase heat to high and boil until slightly thickened, 1-2 minutes. Turn off heat, then whisk in cheese. Season with salt (we used 1/2 tsp; use 1 tsp for 4) and pepper.

Chop bacon into small pieces. Stir bacon, cavatappi, and cauliflower into pan with sauce. TIP: If sauce seems dry, add remaining reserved cooking water, 1 TBSP at a time, until creamy.

Divide mac ’n’ cheese between plates. Garnish with scallion greens and serve.