INGREDIENTS
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1 1 pound loaf challah or brioche, cut into 3/4-inch cubes
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3 tablespoons unsalted butter, plus more for dish
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1 pound assorted mushrooms (such as cremini, oyster, shiitake), cut into 1/4- to 1/2--inch slices
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1 tablespoon dry sherry (optional)
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4 medium leeks, halved lengthwise, then cut into 1/4-inch slices
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2 garlic cloves, minced
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1 1/4 teaspoons kosher salt
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1 teaspoon chopped fresh thyme
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6 large eggs
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2 cups heavy cream
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1 cup whole milk
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3/4 cup shredded Swiss cheese
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1/4 cup freshly grated Parmesan
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1/2 teaspoon ground pepper