Preheat oven to 400 degrees F.
In a saucepan, cook the leeks in 2 tablespoons of the butter over medium heat, stirring, until they are softened. Add the flour and cook the mixture over low heat, stirring for 3 minutes. Add the half-and-half, bring the liquid to a boil, and simmer the mixture, stirring occasionally, for 5 minutes. Stir in 1/4 cup each of the Parmesan and Gruyere, mustard, nutmeg, and salt and pepper, to taste.
In a separate saucepan combine the potatoes with enough water and salt to cover them by 2-inches, bring the water to a boil, and simmer the potatoes for 1 minute. Drain the potatoes.
Transfer the potatoes to a buttered 1 1/2-quart baking dish. Spoon the leek mixture over the potatoes, sprinkle it with the remaining cheeses, and dot the top with the remaining butter. Bake the gratin for 35 to 40 minutes, or until the potatoes are tender and the top is golden brown.
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