Unsalted butter for preparing the tart pan
1 pound (500 g) onions, peeled
3 tablespoons (1 1/2 ounces; 45 g) unsalted butter
1 tablespoon fresh thyme leaves, carefully stemmed
Sea salt and freshly ground black pepper to taste
Freshly grated nutmeg to taste
4 large eggs
1/4 cup (60 cl) whole milk
3 tablespoons heavy cream

1. Preheat the oven to 425 degrees F (220 degrees C.; gas mark 7/8)
2. Generously butter the bottom and sides of the baking dish. Set aside.
3. Slice the onions in half lengthwise. Place, cut side down, on a cutting
board and slice crosswise into very thin slices.
4. In a large unheated skillet, combine the onions, butter, rhyme, salt,
pepper, and nutmeg. Sweat over moderate heat, covered, until
onions are soft, about 8 minutes. They should not caramelize or
turn brown. Taste for seasoning. Set aside.
5. Crack the eggs into a medium-size bowl and whisk just to blend.
Whisk in the milk and cream.
6. Transfer the onions to the prepared baking dish, smoothing them out
with the back of a spoon. Pour the egg mixture over the onions.
Season with additional pepper and nutmeg. Place in the center of
the oven and bake until the top is a deep golden brown and the
custard is firm, about 30 minutes. To test for doneness, insert the
tip of a knife in the center of the quiche. It is done when the knife
comes out clean. Do not underbake or the quiche will be mushy, not
firm. Let sit for about 5 minutes to firm up. Serve warm, cut into
thin wedges.