Ombré Gratin

Preparation

1. Step 1

   Set a rack in the middle of the oven and another right below it. Heat the oven to 375 degrees. Grease a 13-by-9-inch baking dish with butter; set aside.

2. Step 2

   Prepare the cream: In a large saucepan, melt the butter over medium-low. Add the shallots, garlic and black peppercorns and cook, stirring occasionally, until shallots are softened, about 5 minutes. Stir in the half-and-half, bay leaves, thyme, nutmeg and 1½ teaspoons salt and bring to a simmer over medium-high. Reduce the heat to maintain a low simmer and cook, whisking occasionally, until slightly thickened, about 20 minutes.

3. Step 3

   Strain the seasoned cream through a fine-mesh sieve into a large bowl. (You should have about 3 cups.) Return the cream to the saucepan off the heat. In the same bowl, whisk the egg yolks, then whisk in a few tablespoons of the warm cream, 1 tablespoon at a time, to temper the yolks. (Tempering prevents the cold yolks from scrambling when combined with the warm cream. The yolks will help your filling hold together.) Whisk the egg yolk mixture into the cream. Reserve and refrigerate 1 cup cream for serving. Rinse and dry the bowl.

4. Step 4

   Using a mandoline (or very sharp knife; see Tip), slice the unpeeled red potatoes crosswise ⅛-inch thick. In the bowl, toss the potatoes with 1½ teaspoons salt and ¾ teaspoon pepper until evenly seasoned, then stir the potatoes into the cream in the saucepan. (The warmth of the cream will help the potatoes start to cook.) Set aside.

5. Step 5

   Peel, trim and halve the beets, then slice them ⅛-inch thick using a mandoline (or very sharp knife; see Tip). Add the beets to the bowl and toss with ¾ teaspoon salt and ¼ teaspoon pepper. Shingle the sliced beets in the baking dish, overlapping slightly, in two even layers, then sprinkle them evenly with 1⅓ cups grated Gruyère. Wipe out the bowl to remove any beet juices.

6. Step 6

   In the bowl, season the sliced sweet potatoes with 1 teaspoon salt and ¼ teaspoon pepper; toss to coat. Shingle them on top of the cheese-topped beets in 2 to 3 even layers, then press them gently to compress the vegetables evenly, eliminating any air pockets. Sprinkle the sweet potatoes with 1⅓ cups grated Gruyère.

7. Step 7

   In the bowl, season the squash with ¾ teaspoon salt and ¼ teaspoon pepper; toss to coat. Shingle the squash over the cheese-topped sweet potatoes in 2 even layers, press to compress the mixture, then sprinkle the squash with the remaining 1⅓ cups grated Gruyère.

8. Step 8

   Shingle the potatoes on top in 2 to 3 even layers, then slowly pour the remaining cream mixture evenly over the sliced vegetables. Set on the middle rack of the oven and place a large sheet pan directly underneath on the rack below to catch any drips. Bake until the vegetables start to soften, about 1 hour.

9. Step 9

   After the vegetables have baked for about 40 minutes, prepare the phyllo: In a small saucepan, melt the butter over medium-low heat. Working on a large, clean surface, set 1 sheet of phyllo down. Brush lightly with melted butter then set a second sheet of phyllo on top. Take your hands and scrunch the phyllo into a 5½-inch rosette with lots of folds. Push aside, then repeat with remaining 14 sheets of phyllo, creating 8 rosettes total.

10. Step 10

    After the sliced vegetables have cooked for 1 hour, remove from the oven. Increase heat to 400 degrees. Using a paper towel, gently pat the surface to remove any liquid that may have risen to the surface, then cover the top with the 8 phyllo rosettes, scrunching them as needed to fit. Brush the phyllo generously with melted butter.

11. Step 11

    Bake until the phyllo is crisp and golden and the vegetables are tender and a paring knife slides easily through the center, about 30 minutes. Let cool for 20 to 30 minutes before slicing. (Patience is crucial; if you slice this before the liquids are reabsorbed, the gratin won’t form slices without slipping apart.)

12. Step 12

    While the gratin firms up, reheat the reserved cream over medium until warmed through, about 2 minutes. Season the warm sauce with salt and pepper.

13. Step 13

    To serve, cut the gratin in half lengthwise, then cut crosswise to form 12 even slices. Pass the sauce at the table, to spoon on the side.