crust

1. Combine flour, sugar and salt in the bowl of a food processor fitted with a blade attachment. Pulse until the butter is pea sized. Add 3 tablespoons of ice water and pulse until the dough just comes together when squeezed, adding up to 3 more tablespoons of ice water if needed. Turn out dough onto a lightly floured surface (it will be crumbly) and shape into a fat disk. Wrap in plastic wrap and refrigerate 1 hour, or until firm.
2. On a lightly floured surface, roll out dough to a 13-inch round. Carefully transfer dough to a 10 or 11- inch tart pan with removable bottom and trim edges. Prick the bottom with a fork and store in the freezer while the oven preheats.
3. Preheat oven to 350°F.
4. Place the tart pan on a baking sheet. Line the dough with foil and fill with pie weights or dried beans. Bake until the edges are just lightly golden, 15- 20 minutes. Remove the foil and weights and continue baking until the crust is lightly golden all over, 15- 20 more minutes. Transfer to a rack to cool.

filling

1. Heat butter in a large skillet over medium heat. Add leeks, season with salt and pepper, and slowly sauté until soft and translucent, stirring occasionally, 10- 15 minutes. Transfer leeks to a large bowl.
2. In the same pan, add olive oil and raise heat to medium-high. Add swiss chard, season with salt and pepper, and sauté 4-5 minutes until wilted and tender. Remove from heat and when cool enough to handle, squeeze out excess liquid, finely chop, and add to bowl with leeks.
3. Stir in parsley, cream, eggs, pecorino, a pinch of salt and pepper. Spread filling evenly into prepared tart shell and bake 25- 30 minutes, until filling is puffed and firm in the center.
4. Serve warm or at room temperature.