Make the pastry shell: In a large chilled bowl combine the flour, salt, chilled butter, and the lard or vegetable shortening. Blend together until the mixture resembles coarse meal.
Pour 3 tablespoons of ice water over the mixture. Toss together lightly and gather the dough into a ball. Add more water until dough holds together. Wrap in plastic and refrigerate at least 1 hour before using.
Preheat the oven to 400 degrees.
On a lightly floured surface, roll the dough into a 14-inch round, about 1/8-inch thick. Place the dough in a 9-inch tart pan. Gently press it into the bottom and around the sides of the pan, being careful not to stretch the dough. Trim off the excess dough by rolling the pin over the rim of the pan.
Line the inside of the tart shell with aluminum foil and fill with beans or pie weights. Blind bake in the middle shelf of the oven for 10 minutes. Remove foil, prick the dough all over with a fork and return to the oven for an additional 10 minutes, or until the shell begins to brown.
Make the filling: Preheat the oven to 350 degrees. In a heavy 6 to 8-inch skillet, heat the oil over moderate heat until haze forms above it. Add the onions and cook for about 5 minutes, stirring frequently, until they are soft and transparent but not brown. Stir in the paprika and set aside off the heat.
In a bowl toss together the Gruyere and Emmentaler cheese until thoroughly combined. Spread half the cheese evenly in the baked pastry shell and scatter the onions over it. Cover with the remaining cheese.
Beat the eggs cream, milk, salt, and nutmeg together with a wire whisk and pour the mixture slowly and evenly over the cheese.
Bake in the upper third of the oven for 10 minutes. Increase the heat to 425 degrees and bake for an additional 15 minutes, or until the filling has puffed and browned and a knife inserted in the center comes out clean. Remove pan from the oven and remove the tart from the pan.
Serve hot or at room temperature as a first or main course.