Place oven rack on highest level. Preheat oven to 375 degrees F (190 degrees C). Set prebaked tart shell on a baking sheet (see Editor's Note).
Chop the clams finely, and reserve 1/2 cup of the liquid. Beat the eggs well, and add clam liquid, cream, nutmeg, salt, and pepper. Whisk until smooth, and stir in the clams.
Fry the bacon over moderate heat until crisp, and then remove from pan and set aside. Leave about half a tablespoon of bacon grease in the pan, and add onion. Cook and stir until the onion has softened and turned translucent, about 5 minutes. clam mixture. Crumble the bacon into mixture, then pour into mixture into pastry shell.
Crumble the bacon into the tart shell; add onion. Carefully pour custard into shell and transfer to preheated oven.
Bake on the top rack until center is set, or until a knife inserted in center comes out clean, about 25 minutes. Serve hot or warm.