INGREDIENTS
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Softened butter
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One 1¼-pound loaf bakery white bread—crusts removed, bread sliced (about 18 slices)
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¼ cup grainy mustard
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1 pound thinly sliced Virginia ham
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2 tablespoons chopped tarragon, plus more for garnish
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¾ pound Gruyère cheese, coarsely shredded (3 cups)
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3 cups milk
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4 large eggs
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Freshly ground black pepper