INGREDIENTS
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4 tablespoons ( 1/2 stick) unsalted butter, plus more for greasing
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10 slices white country bread, halved
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1/4 cup all-purpose flour
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3 cups whole milk
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2 tablespoons Dijon mustard
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1 pinch Kosher salt and freshly ground black pepper, more to taste
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1/2 pound hinly sliced ham, such as Virginia or Black Forest, coarsely chopped
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8 ounces Gruyère cheese, shredded (2 cups)
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6 large eggs