Lightly grease a 9-in springform pan with 3-in sides. Set aside.

Make the two batches of Foolproof Shortcrust Pastry, one after the other. When the first one is done, dump it on a floured working surface, and make the second batch right away. Dump it over the first batch, then, using your hands, gather the whole thing into a loose, flattened ball. Sprinkle the dough with flour, then roll out to a 16-in circle. The pastry should be about ¼ inch thick.

Carefully fold the dough in half, then place over the bottom half of the prepared springform pan. Unfold, then gently ease the dough into the pan, pressing it onto the bottom and creasing the dough to flatten it against the sides of the pan. Using a fork, prick the bottom pastry all over.

If blind baking with pie weights or dry peas, cut off the overhanging pastry so the pastry is flush with the top of the pan. If blind baking without pie weights or dry peas, let the pastry hang over the sides of the pan. Freeze the pastry-lined pan for at least 30 minutes, or until ready to blind bake.

Preheat the oven to 350°F (175°C).

If blind baking with pie weights or dry peas, cut out a large piece of parchment paper and ease it into the pastry to line it. Fill the pan with pie weights or dry peas. Bake for 30 minutes. Remove from the oven. Carefully lift the parchment with the weights or peas out of the pan, then return the pan to the oven and bake for an additional 15 minutes, or until the crust looks dry and is light golden brown. Set aside on a cooling rack.

If blind baking without pie weights or dry peas, place the pan with the overhanging pastry straight from the freezer to the oven. Bake for 45 minutes, or until the crust looks dry and is light golden brown. Set aside on a cooling rack.

In a large saute pan or Dutch oven set over medium heat, warm up the extra-virgin olive oil until shimmering. Add the sliced onions, sprinkle with 1 tsp of the salt, then stir to combine. Cover and cook for 15 minutes. Uncover, stir, reduce the heat to low, and continue cooking, uncovered, until the onions are tender and lightly browned, about 1 hour. Stir every 15 minutes to ensure the onions cook evenly. Transfer the cooked onions to a large bowl and let cool completely.

In a skillet set over medium-high heat, cook the bacon until crisp but not dry. Using a slotted spoon, transfer the bacon to the bowl with the cooked onions and let cool.

Preheat the oven to 325°F (160°C).

Make the custard: In a large bowl or an 8-cup measuring cup, combine the milk, cream, eggs, remaining salt, pepper, and nutmeg and, using a hand mixer, blend until very frothy.

Place the springform pan with the blind baked crust over a baking sheet to catch any leaks. Mix the cooked onions and bacon to combine, then spread half of the mixture over the bottom of the crust. Pour half of the custard over the onions. Sprinkle with half the grated cheese. Spread the remaining onion and bacon over. Blend the remaining custard again to refroth, then pour into the crust. Sprinkle with the remaining cheese.

Bake for about 90 minutes, or until the top of the quiche is golden brown and the center is just set. The center of the Quiche Lorraine should still jiggle a bit: it will set fully as the quiche cools.

Transfer the Quiche Lorraine to a wire rack and let cool to room temperature, at least 2 hours. Refrigerate until the quiche is fully chilled, at least 3 hours, or ideally overnight. Refrigerating the quiche will allow it to fully set and slice more cleanly. You can keep the quiche refrigerated for up to 3 days.

If you blind baked the crust without pie weights or dry peas, using a sharp knife to cut off the extra crust along the rim.

To unmold the Quiche Lorraine, slide a sharp knife along the edge of the ring mold or cake pan to loosen the quiche, then loosen the springform and remove the quiche from the pan.

Slice the Quiche Lorraine into portions, then serve cold or room temperature. You can also reheat portions in a 350°F oven for 10 minutes. Serve the quiche with a lightly dressed green salad.

Refrigerate leftover Quiche Lorraine in an airtight container for up to 3 days.