Instructions

1. For the crust: Combine flour, cornstarch, and salt in a large bowl. Using a pastry blender or 2 table knives, work butter into flour mixture until it resembles coarse meal flecked with pea-size pieces of buter, then rub butter pieces into flour mixture with your fingers. Add up to 6 tbsp. ice water, 1 tbsp. at a time, mixing unti la rough dough forms. Knead dough in bowl until it just comes together, then shape into a ball. Flatten dough into a disk, wrap in plastic wrap, and refrigerate for at least 30 minutes and up to 8 hours.
2. Preheat oven to 400º. Roll dough out on a lightly floured surface into a 14" round. Ease pastry into a 2" deep 10" round loose-bottom tart pan and crimp edges. Prick dough all over with a fork, line with parchment paper, and add pie weights or dried beans. Bake until crust is set and edges begins to color, 30-35 minutes. Remove parchment paper and weights and continue to bake crust until lightly browned, about 5 minutes more. Remove crust from oven and set aside. Reduce oven temperature to 375º.
3. For the filling: Melt butter in a large skillet over medium heat. Add onions and cook until soft, 6-8 minutes. Transfer to a large bowl. Put cheeses into a food processor and pulse until smooth. Add eggs and milk and pulse until blended. Transfer to bowl with onions. Add chard, thyme, nutmeg, and salt and pepper to taste and mix well.
4. Pour filling into crust and bake until set, 40-45 minutes. Serve quiche at room temperature.