Preheat oven to 375 F

Bring a large (5-6 qt) pot of water to a boil, and add about 1 tsp kosher salt. Add the cauliflower, and boil until just cooked through, but not too soft, about 5-7. minutes. Remove cauliflower with a slotted spoon or spider, and set aside. 

With the water still boiling, add the macaroni to the same pot of water, and cook according to the package directions. Drain and return to the pot. 

While the cauliflower and pasta are cooking, start making the sauce. In a smaller (3-4 qt pot), heat the olive oil over medium heat. Add the flour, and stir, cooking for about 1 minute until it just begins to turn golden. Slowly whisk in milk, making sure to whisk out any lumps. Reduce heat to medium-low, and whisk frequently until thickened, about 7-10 minutes. 

Over low heat, add cheese to the sauce, one handful at a time, and whisk thoroughly before adding more. Add nutmeg and garlic, turn off heat, and stir to thoroughly combine. Add salt to taste. 

Add the cauliflower to the pot with the pasta. Add the cheese sauce, and fold in to combine. 

Pour mixture into a greased 12 inch cast iron skillet or similar sized baking dish. In a small bowl, mix bread crumbs, parsley, and scallions. Sprinkle mixture evenly over the top of the cauliflower macaroni dish. Spray the top lightly with cooking spray. 

Cover in foil, and bake for 40 minutes. Remove foil, and bake an additional 20 minutes to brown slightly on top. 

Serve in a bowl or on a plate. Pairs well with a light side salad.