Preheat the oven to 375°F (190°C).

In a mixing bowl, combine flour, salt, and butter. Mix until the mixture resembles coarse crumbs. Add ice water gradually until the dough forms.

Roll out the dough and press it into a tart pan. Trim the edges and poke holes in the bottom with a fork.

In a skillet, heat olive oil over medium heat. Add the sliced leeks and sauté until soft.

Add the chopped Swiss chard to the skillet and cook until wilted. Remove from heat and let cool slightly.

In a large bowl, whisk together the eggs, heavy cream, Pecorino cheese, black pepper, and nutmeg.

Add the sautéed leeks and Swiss chard mixture to the egg mixture and stir to combine.

Pour the filling into the prepared tart shell and bake for 30-35 minutes or until the filling is set and the top is golden.