We replaced pasta with cauliflower in this creamy low-carb "mac" and cheese. A sprinkle of Parmesan on top and a quick trip under the broiler gives this cheesy side dish a savory, crispy crust.
Position rack in upper third of oven. Preheat broiler to high.
Bring a large pot of water to a boil. Add cauliflower and cook until tender, about 5 minutes. Drain.
Meanwhile, heat 1 1/4 cups milk in a large ovenproof skillet over medium heat until steaming. Whisk the remaining 1/4 cup milk, cornstarch and dry mustard together in a small bowl until smooth. Add to the hot milk and cook, whisking constantly, until the sauce bubbles and thickens, about 2 minutes. Remove from heat and whisk in pepper and salt, then whisk in Cheddar and cream cheese until melted and smooth. Add the cauliflower and stir to coat. Sprinkle Parmesan evenly over the top and broil until browned in spots, 3 to 5 minutes. Garnish with chives, if desired.