Preheat oven to 350 degrees. Carefully and thoroughly clean leeks. Finely chop tender parts of the leeks. Melt butter in a large saucepan. Cook leeks over medium heat for about 3 minutes, until soft. Blend in flour and cook for 1 minute. Sir in milk. Bring to a boil, then reduce heat and simmer for 3 minutes. Remove from heat.
When cool, beat in Roquefort cheese. Add eggs, nutmeg, and salt and pepper. Make sure to taste and adjust seasoning because Roquefort can be salty and additional salt may not be needed.
Spoon mixture into crust. (If there is extra filling, place in lightly buttered ramekins and cook alongside the tart.) Bake for 40 minutes. Filling should be golden brown and set. Cool on rack for a few minutes before serving. Alternatively, reheat the following day when it is even better.
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