Place a rack in the middle of the oven; preheat oven to 350°.
In a large skillet over med-low heat, melt the butter.
Add the leeks and onion and stir constantly.
Cook until soft, 8-10 minutes.
Remove from the heat and set aside to cool.
In a small bowl, whisk the egg, cream, salt, pepper, and nutmeg together.
Stir the cooled onion mixture into the egg mixture.
Divide among the tartlet shells.
Place the tarts on a baking sheet and bake until the custard sets, about 10-15 minutes.
Serve the tartlets warm or at room temperature.
**May add crab--1/4 cup picked over lump crabmeat, coarsely chopped; add a scant 1/4 teaspoon crabmeat to each phyllo shell before you spoon in the custard mixture; bake as directed; top with creme fraiche and a touch of caviar, if you like.