Grind the poppy seeds in a mill or coffee grinder.

Combine the milk, margarine, and sugar in a saucepan. Cook on low heat, stirring often, until the sugar dissolves. Gradually pour about half of the hot milk into the beaten eggs, whisking constantly. Return the egg and milk mixture to the saucepan.

Continue to cook and stir until the mixture begins to thicken and coats the back of a metal spoon. (Run your finger down the coated spoon: it should draw a clear line.) Add the poppy seeds and stir well to blend.

Remove from heat; cool before using. Store unused filling in the refrigerator for up to five days.