Step 1

Preheat oven to 375°F. Coat a 9-by-13-inch baking dish with cooking spray.

Step 2

Melt butter in a large saucepan over medium heat. Add shallots and cook until tender and lightly browned, about 5 minutes. Add garlic, mustard, nutmeg, pepper and salt and cook until fragrant, about 1 minute. Add potatoes, parsnips, broth and half-and-half and bring to a boil. Adjust heat to maintain a simmer and cook until thickened, about 5 minutes.

Step 3

Spread the mixture evenly in the prepared baking dish. Press down on the vegetables to submerge them in the sauce. Cover with foil.

Step 4

Bake until a knife inserted in the center meets no resistance, about 35 minutes. Increase oven temperature to 425°. Uncover and bake until the top is golden and bubbling, about 15 minutes more. Let stand for 15 minutes before serving.