Gratin of Potatoes and Parsnips

Ingredients

1 tablespoon unsalted butter, softened
3 pounds red potatoes, unpeeled, cut into 1/4-inch slices
2 pounds parsnips, peeled and cut into 1/4-inch slices
1 large red onion (about 3/4 pound), peeled, cut in half, then cut into 1/4-inch slices
1/2 cup unsalted butter, melted
1-1/2 teaspoons salt
Freshly ground black pepper
3 tablespoons freshly grated Parmesan cheese
4 cups light cream
1-2 cups chicken stock (if needed)

Directions

Preheat the oven to 350 degrees. Grease a large (9x12-inch) baking dish with the tablespoon of softened butter. Arrange one-third of the sliced potatoes in the bottom, followed by half the parsnips. Scatter half the onions over the top. Drizzle with a third of the melted butter, and sprinkle with 1/2 teaspoon salt and a generous grinding of pepper. Repeat. Finish with a final layer of potatoes, arranged evenly; drizzle on the remaining butter. Top with 1/2 teaspoon salt, pepper to taste, and the cheese. Pour on the cream. It should come just to the surface of the potatoes. If it does not, add up to 2 cups chicken stock.
Bake, covered, for 45 minutes. Remove the lid and bake 15 minutes longer, or until brown and bubbling