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Start by making the béchamel sauce: Melt the butter in a saucepan over low heat.
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Add the flour, and mix briskly with a whisk for 2 minutes. Still over low heat, add 2 cups of the milk, Continuing to whisk briskly. When the sauce comes to a boil, season with salt, pepper and nutmeg.
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Remove from the heat, and let cool.
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Spread butter on one of the side of the bread (will be bottom side)
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Spread mustard on the other side (top)
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Top each slice of bread with a slice of ham and then liberally with the béchamel sauce.
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Sprinkle with the grated gruyere, and cook in an oven or under a broiler until golden brown.
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Serve with pomme frites or salad