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1
Scrub and pierce potatoes and bake at 400 degrees F. until tender, 50 to 60 minutes.
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2
Cover with an inverted bowl to keep warm while preparing the sauce.
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3
While potatoes bake, melt butter in a medium saucepan over medium heat.
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4
Saute onion till tender.
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5
Whisk in flour and continue cooking and stirring for a minute or two.
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6
Gradually whisk in the milk and then the cream.
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7
Continue stirring while bringing mixture just to a boil.
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8
Lower heat and barely simmer for 5 or 6 minutes.
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9
Remove from heat.
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10
In a small bowl, whisk egg yolks.
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11
Temper yolks by drizzling in a bit (1/4 cup) of the hot mixture into them while whisking.
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12
Add the tempered yolks to the rest of the sauce and return the saucepan to low heat.
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13
Cook until mixture thickens, about 1 minute.
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14
Stir in lemon juice, cayenne and nutmeg.
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15
Reserve about 1/3 cup Swiss cheese and add remaining cheese to sauce, stirring until cheese melts.
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16
Stir in the ham and heat through, about 2 minutes over low heat.
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17
Remove from heat.
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18
Taste and adjust seasonings, if needed.
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19
Preheat the broiler.
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20
Split potatoes lengthwise and place in a large casserole (11' X 13') Flake the potato flesh with a fork and season lightly with salt and pepper.
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21
Spoon sauce over the potatoes, distributing evenly.
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22
Sprinkle with reserved Swiss cheese and broil until heated through and just starting to brown.
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23
Serve.
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24
This can be prepared ahead and refrigerated, covered well, for up to 24 hours.
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25
Bake in a 375 degree oven for 40 to 50 minutes until hot and starting to brown.
