Ham-and-Swiss Quiche

Preheat oven to 400 degrees, 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.

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.

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.