Root Vegetable Gratin

From Smitten Kitchen - visit the page for lovely photos and comments from people who tried the recipe


Ingredients
Olive oil
1 large onion, sliced (1 1/2 cups)
2 cups (1/4-inch-sliced) fennel, top and core, if thick, removed
3 cloves garlic, minced (1 tablespoon)
1 pound sweet potatoes, peeled and sliced 1/4-inch thick
1 pound celery root, peeled and sliced 1/4-inch thick
1 pound Yukon gold potatoes, peeled and sliced 1/4-inch thick
1 cup heavy cream
2 cups chicken or vegetable stock
2 cups grated gruyere cheese (6 ounces with rind)
2 teaspoons minced fresh thyme leaves
Kosher salt and freshly ground black pepper
2 1/2 cups coarse fresh breadcrumbs or panko
2 to 3 tablespoons melted butter

Method
Heat your oven to 350°F. Butter a 13×9-inch deep baking dish.

Heat 2 tablespoons olive oil in a large deep (ideally 12-inch) saute pan over medium heat and add the onions and fennel. Cooking, tossing occasionally, until lightly browned and tender, about 10 minutes. Add the garlic and cook for one minute more.

 Meanwhile, in the largest bowl you own, combine the onion mixture with remaining vegetables, cream, stock, cheese, thyme, 1 tablespoon kosher salt and 1 1/2 teaspoons black pepper. Pour mixture into prepared dish and spread until even. Mix breadcrumbs and butter until evenly coated and distribute evenly over top of dish.

 Bake 1 1/2 hours uncovered, or until vegetables are very tender when tested with a small knife and the top is browned and bubbly. Allow to set for 15 minutes at room temperature and serve hot.

Make Ahead

To make this ahead of time,  assemble and bake 1 to 3 days ahead and store in fridge.  Let set on the counter for an hour before going in the oven, bake as described above.