Preheat the oven to 400°F. Bring a large pot of water to boil. 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 in the oven until tender, about 25 minutes.
2. Step
Meanwhile, heat a large pan over medium high heat. Add bacon and cook until just crispy. Remove from the pan and set aside to drain on paper towels. Remove pan from heat, reserving just 1 tbsp of bacon fat.
3. Step
Once water is boiling, add pasta and cook until al dente. Drain.
4. Step
Heat butter in a pan with reserved bacon fat until melted. Add scallion whites and cook for one minute. Add flour and cook, stirring, until lightly browned about 1 minute. Slowly whisk in milk. Stir in stock and simmer until thickened. Stir in cheese until thoroughly combined. Remove pan from heat. Season with salt and pepper.
5. Step
Crumble bacon into pieces. Stir bacon, pasta, and ¾ of the cauliflower into sauce. Transfer into a lightly oiled baking dish. Scatter remaining cauliflower on top. Bake until bubbly, 5-7 minutes.
