Blanching the potatoes for a gratin ahead of time cuts the baking time considerably and ensures that the potatoes will be cooked through. To make fresh bread crumbs, cut two slices of bread into large cubes and process them in a mini-food processor or blender for 10 to 15 seconds. This dish can be prepared in advance.
- 1 1/4 cups half-and-half (may substitute nonfat half-and-half)
- 5 sprigs thyme, tied together with kitchen twine
- 1 clove garlic, chopped
- 1 large (12 ounces) russert potato peeled, cut into 1/4-inch slices and placed in cold water to prevent discoloration
- 1 medium (12 ounces) sweet potato, peeled and cut into 1/4 inch slices
- 1 small leek, white and tender green parts, cut into 1/2 dice (about 1/2 cup)
- 1/2 teaspoon salt
- 1/4 cup freshly ground black pepper
- 1/4 cup grated Parmigiano-Reggiano cheese
- 1 cup fresh bread crumbs, preferably brioche
Lightly grease a 7- or 8-inch casserole dish with nonstick cooking oil spray. Set aside.
In a medium saucepan over medium to medium-low heat, combine 1 cup of the half-and-half with the thyme and garlic, and heat until a few bubbles form (do not scald the milk). Drain the russet potato slices well and add them to the pan. Cook for 12 to-- minutes or until they are just tender. Use a slotted spoon to transfer to a large bowl. Add the sweet potato and chopped leeks to the pan and cook for 6 to 8 minutes, stirring occasionally, until they are just tender. Remove the pan from the heat; discard the thyme bundle. Transfer the sweet potato and leeks to the bowl, along with any remaining liquid from the pan. Add the remaining 1/4- cup half-and-half, then toss everything together lightly and sprinkle with salt and pepper before spooning it into the prepared dish. Top with the Parmigiano-Reggiano cheese and bread crumbs.
At this point, the gratin can be covered with plastic wrap and refrigerated for up to 1 day. Remove from the refrigerator at least 1 hour before baking.
Preheat the oven to 450 degrees. Uncover the gratin and bake for 15 to 18 minutes or until it is warmed through and the top is golden brown.
Adapted from a recipe by David Hagedorn.
Tested by Michael Taylor.
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