Wash and dry all produce. Adjust rack to middle position and preheat oven to 400 degrees. Bring a large pot of salted water to a boil. Trim, then thinly slice scallions, keeping greens and whites separate. Toss cauliflower on a baking sheet with a large drizzle of olive oil and a pinch of salt and pepper. Roast in oven until tender, 20-25 minutes.

Once water is boiling, add cavatappi to pot. Cook, stirring occasionally, until al dente, 9-11 minutes. Drain.

Meanwhile, heat a large pan over medium-high heat (use an ovenproof pan if you have one). Add pancetta and scallion whites. Cook, tossing, until crisp and browned at the edges, 4-5 minutes. Remove from pan with a slotted spoon, keeping as much oil in pan as possible.

Melt 2 TBSP butter in same pan over medium heat, then add flour. Stir constantly until flour loses its raw smell, 1-2 minutes. Slowly pour in 2 cups milk (we sent more), whisking to combine. Stir in stock concentrates and let simmer until thickened, 1-2 minutes. Remove pan from heat, then stir in cheese. Season with salt and pepper.

Stir cavatappi, pancetta mixture, and 3/4 of the cauliflower into sauce in pan. (TIP: If your pan is not ovenproof, transfer mixture to a lightly oiled baking dish at this point.) Scatter remaining cauliflower over top. Bake in oven until bubbly, 5-7 minutes.

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