Make the crust (if you choose to make it from scratch):

In a food processor:

• Mix flour with salt and place them in the food processor.
• Add cold butter cut into small cubes, and pulse a few times until you obtain a mixture that resembles sand or bread crumbs. Do not overmix.
• Add water, one tablespoon at a time, until the mixture holds together.

By hand:

• Mix flour and salt together, then cut the cold butter into small cubes and add it to the flour.
• Use a pastry tool to incorporate the butter into the flour, or use your fingertips. You should have a mixture that resembles sand or bread crumbs.
• Add water, one tablespoon at a time, until the mixture holds together.
• Divide the dough into two parts, then shape each half into a disk. Wrap each half in plastic foil and refrigerate until firm, about 1 hour or up to 1 day. (You can skip this part if you are in a hurry).

  Note: For this recipe, use only one disk. Place the other one in the freezer and use it later for a different recipe.

Prebake the crust:

• Roll out the dough on a lightly floured surface into a 12-inch circle. Fit into a 9-by-1 ½-inch round tart pan (or 9-inch pie dish), pressing dough into corners, extending the edges extra high to allow for shrinkage. Transfer to freezer to chill for 30 minutes. 

• Fill at least two-thirds with baking weights—dried beans, rice, ceramic, or metal pie weights. 

• Bake first for 15 minutes, remove from oven and let cool a few minutes. Carefully remove aluminum foil and weights.
• Poke the bottom of the pie pan with a fork and return to the oven.
• Bake for an additional 10 minutes or until lightly golden. (Fork holes are for any air to escape.) Transfer to a wire rack to cool while making the filling.

Make the filling:

• Melt the butter in a heavy-based frying pan over medium heat. Add the onions and cook, frequently stirring to avoid burning, for 30 minutes or until well-browned and caramelized. Remove the onions from the pan and set them aside.
• Preheat the oven to 375F/190C.
• Beat the eggs in a large bowl. Stir in the heavy cream and season to taste with salt and pepper. Add the Cheddar cheese and mix well. Stir in the cooked onions.
• Pour the egg and onion mixture into the baked pastry case and sprinkle with the Parmesan cheese.
• Place tart pan on a baking sheet and bake in the preheated oven for 15-20 minutes, until the filling has set and is beginning to brown.
• Remove from the oven and leave to rest for at least 10 minutes.
• Cut the tart into slices and serve hot or at room temperature.