Wash and dry all produce. Preheat oven to 400 degrees. Bring a large pot of salted water to a boil. 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. Reserve 1/4 cup pasta cooking water, then drain.

Meanwhile, heat a large pan over medium-high heat (use an ovenproof pan if you have one). Add bacon and cook, turning occasionally, until just starting to turn crispy, 7-9 minutes. Turn off heat. Transfer bacon to a paper towellined plate. Pour out all but 1 tsp grease in pan.

Melt 1 TBSP butter in same pan over medium heat. Add scallion whites and cooked until softened, about 1 minute. Add flour and stir constantly until lightly browned, 1-2 minutes. Slowly whisk in milk and reserved pasta cooking water, then stir in stock concentrate. Simmer until thickened, 1-2 minutes. Remove pan from heat, then stir in cheese. Season with salt and pepper.

Crumble bacon into small pieces using your hands. Stir bacon, cavatappi, and 3/4 of the cauliflower into sauce in pan. (TIP: If your pan is not ovenproof, transfer mixture to a medium, lightly oiled baking dish at this point.) Top with remaining cauliflower. Bake in oven until lightly browned, 5-7 minutes.

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