The side dishes on our annual Thanksgiving menu almost always include mashed potatoes and roasted root vegetables. This year I decided to change up the routine by combining the two, adding a little cream and topping off the hybrid with some crunch. It's heartier that way and tastier, too.
This recipe calls for a pound of turnips and one-half pound each of carrots and parsnips. What if you're not a fan of turnips? (Some folks find them a bit funky.) Just leave them out and increase the amount of carrots and parsnips. But try not to mess with the specified amount of potatoes. It's their starch that thickens the sauce.
The dairy is a combination of cream and milk (the latter lightens the dish) infused with garlic, thyme and bay leaf. Seasoning a gratin is usually hit or miss – you sprinkle some salt willy-nilly on top of the raw vegetables as you layer them into the baking pan. Here we call for an exact measure of salt to be added to and dissolved into the cream/milk mixture. That way the seasoning is perfect.
It's important to make sure the vegetables are sliced thinly and evenly so that they all become tender at the same time. You can do it by hand, but you will get more uniform results with a mandoline or the slicing disk of your food processor. (If you do choose to work with the mandoline, be sure to put the guard into place.)
One final note – not all baking pans perform the same way in the oven. When I made this dish in a metal pan, the vegetables were tender in 40 minutes. When I used a glazed earthenware pan, they took over 50 minutes. The difference speaks to the relative ability of each material to conduct – or resist – heat. In either case, it's crucial to keep an eye on the process.
Root Vegetable Gratin
13/4 cups whole milk
2 cups heavy cream
4 large sprigs fresh thyme, crushed with a rolling pin
1 bay leaf
4 garlic cloves, smashed
21/4 teaspoons kosher salt
1/2 teaspoon freshly ground black pepper
1 pound russet potatoes
1 pound turnips
1/2 pound carrots
1/2 pound parsnips
2 ounces freshly grated Parmigiano-Reggiano
3/4 cup panko breadcrumbs
2 tablespoons extra-virgin olive oil
Preheat the oven to 375 degrees; adjust the oven rack to the middle position.
In a medium saucepan, combine the milk, cream, thyme, bay leaf, garlic, salt and pepper; heat the mixture over medium-high heat until bubbles form around the edge. Remove from the heat, cover, and let steep while you prepare the vegetables.
Peel all the root vegetables. Using a mandoline or the slicing disk of a food processor, slice them crosswise, 1/8-inch thick. Remove and discard the thyme, bay leaf and garlic cloves from the cream mixture and pour one-fourth of the mixture into a 9-by-13-inch baking pan. Add the vegetables and the remaining milk mixture to the baking pan (the liquid will just come up to the level of the vegetables). Stir the vegetables to make sure they are separated and then press them down to distribute them evenly. Bake the gratin on the middle shelf of the oven for 25 minutes. Remove the pan from the oven and press the vegetables down with a large metal spatula. Return the gratin to the oven and bake until the liquid has thickened and vegetables are tender when pricked with a knife, about 15 to 25 minutes more.
In a bowl combine the cheese, panko and oil; sprinkle the mixture evenly over the top of the pan. Return the pan to the oven and bake until the top is browned, about 10 minutes.
Cool for 5 minutes before serving.
Makes 10 to 12 servings.