Adapted from Around My French Table by Dorie Greenspan
~from Savoring Time in the Kitchen
1 tablespoon unsalted butter
2 celery stalks, trimmed and cut into a small dice
2 slender leeks, white and light green parts only, quartered lengthwise, washed, and thinly sliced
2 slender carrots, trimmed, peeled, and finely diced
1 medium red bell pepper, cored, seeded, and finely diced
salt and pepper
1 9-inch tart or pie shell partially baked and cooled (watch careful that your crust doesn't contain any cracks or your filling will ooze out. Add a scrap of pastry to the crack and bake a little longer, if necessary)
2/3 cup heavy cream
1 large egg
1 large egg yolk
2/3 cup grated cheese, preferably Gruyère
Melt the butter in a large skillet over medium heat. Add the diced vegetables and saute for about 10 minutes, stirring occasionally, just until barely tender. Season with salt and pepper, then remove the vegetables to a bowl and let cool.
Center a rack in the oven and preheat to 400 F degrees. Put the crust on a baking sheet lined with parchment paper. Spoon the vegetables into the tart shell and even them out. They should almost fill the crust. Whisk the the cream, egg, egg yolk together, season with salt and pepper, and carefully pour over the vegetables. Depending on how your crust and filling, you may have too much custard. Pour in as much custard as you can without it overflowing and wait a few minutes until it has settled, then add more if you can. Very carefully slide the baking sheet into the oven.
Bake the quiche for 20 minutes. Sprinkle the cheese over the top and bake for another 5 or 10 minutes, or until the cheese is golden (the filling should be puffed all over - especially the center), browned, and set. Note: This took a little longer in my oven.
Transfer the quiche to a rack, remove the sides of the pan, and cool until it is only just warm or until it reaches room temperature before serving.