To make the crust:
Place the flour and salt in the bowl of a food processor and pulse for a minute to mix.

Add the egg and cold water and process just until the dough comes together in a ball. Remove it and flatten into a disc.

Wrap in plastic wrap and refrigerate for at least an hour. Roll the dough out on a lightly floured surface and press into an 8-inch pie plate or quiche mold.

Preheat the oven to 400 degrees.

To make the filling:
In a large sauté pan, melt the butter over medium heat.

Add the onions and reduce the heat to low. Cook, covered for about 30 minutes, until soft. Remove the cover and increase the heat to medium. Continue to cook until the onions are caramelized to a golden brown, about 10 minutes. Season with salt and pepper. Turn off the heat. Meanwhile, whisk together the egg yolks and heavy cream and pour the mixture over the onions. Stir to combine.

Pour the onion mixture into the pastry shell. Bake for about 30 minutes, turning the tart once during baking, or until light golden brown on top.

Cool slightly before cutting.