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1
In a large pot, bring the milk and cream to a boil with the garlic.
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2
Season with salt and pepper.
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3
Add the celery root and sweet potatoes and cook, stirring occasionally, until slightly softened, about 2 minutes.
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4
Remove from the heat and let cool for about 20 minutes.
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5
Drain the sweet potato and celery root in a colander set over a large bowl and reserve the cooking liquid.
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6
Meanwhile, in a medium skillet, cook the bacon over moderately high heat until crisp, about 4 minutes.
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7
Drain and transfer the bacon to a medium bowl.
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8
Squeeze out as much of the liquid as possible from the grated onion and apples, then add them to the bacon.
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9
Add the thyme and stir to combine.
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10
Preheat the oven to 350.
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11
Butter a 2 1/2-to 3-quart shallow baking dish.
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12
Arrange one-third of the celery root and sweet potatoes in an even layer.
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13
Spread half of the onion-bacon-apple mixture on top and season lightly with salt and pepper.
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14
Sprinkle with one-third of the cheese.
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15
Repeat the layering once more then top with the remaining celery root and sweet potatoes and the remaining cheese.
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16
Pour enough cooking liquid over the gratin to reach the top of the dish.
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17
Sprinkle with the panko and a buttered sheet of foil.
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18
Bake the gratin in the center of the oven for 45 minutes.
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19
Remove the foil and bake for about 45 minutes longer, until bubbling and golden.
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20
Let the gratin rest for at least 30 minutes before serving.
