1 large yellow onion, halved and thinly sliced
2 tsp olive oil
1 tsp fresh thyme leaves
Coarse salt and pepper
Gluten-free all-purpose flour, for rolling
1 recipe Gluten-Free Pie Dough
5 large eggs plus 2 large egg whites
1/3 cup whole milk
4 ounces ham steak, cut into small cubes (1 cup)
1/2 cup shredded Swiss (2 ounces)
1. Preheat oven to 400°F, with racks in upper and lower thirds. On a rimmed baking sheet, toss together onion, oil, and thyme and spread in a single layer; season with salt and pepper.
2. Lightly flour a rolling pin and work surface and roll out dough to a 12-inch round. Transfer to a 9-inch pie plate, fold overhang under, and crimp edge. (If dough breaks or cracks, patch holes with extra dough.) Line with parchment and fill with dried beans or pie weights. Place onion on top rack and dough on bottom rack; bake until onion is soft and crust is dry at edge and light golden, 15 to 18 minutes. Remove parchment and beans.
3. In a medium bowl, whisk together eggs and egg whites, milk, 1/2 teaspoon salt, and 1/4 teaspoon pepper. Stir in onion, ham, and Swiss. Pour mixture into crust and bake on top rack until center is just set, about 25 minutes. Serve warm or at room temperature.
To store, refrigerate cooled quiche, tightly covered, up to 3 days.