In a mixing bowl, combine flour and salt. Add cold butter and mix until the mixture resembles coarse crumbs.

Add ice water, one tablespoon at a time, mixing until the dough comes together. Form into a disk, wrap in plastic, and refrigerate for 30 minutes.

Preheat the oven to 375°F (190°C). Roll out the dough on a floured surface and fit it into the tart pan. Prick the bottom with a fork.

Blind bake the crust for 15 minutes, then remove from oven and set aside.

In a skillet, heat olive oil over medium heat. Add leeks and sauté until soft, about 5 minutes.

Add Swiss chard and cook until wilted, about 5 minutes. Remove from heat and let cool slightly.

In a bowl, whisk together eggs, cream, Pecorino cheese, black pepper, and nutmeg. Stir in the sautéed leeks and chard mixture.

Pour the filling into the pre-baked tart crust and smooth the top.

Bake the tart in the preheated oven for 25-30 minutes, or until the filling is set and the top is golden.

Remove from the oven and let cool for a few minutes before slicing and serving.