Prepare the spinach. Preheat the oven to 400°F (200°C), conventional heat. Line a half sheet baking pan, about 13 x 18 inches (33 x 46 cm), with parchment paper. Heat a large 12-inch (30 cm) nonstick skillet over medium heat. Add the spinach in batches and cook for 2 to 4 minutes, stirring until wilted. No extra water is usually necessary. Let the spinach cool slightly, then squeeze out as much liquid as you can. Chop it finely. Removing the moisture is what keeps the pastry crisp instead of soggy.

Make the filling. In a large 3-quart (3-liter) mixing bowl, combine the cottage cheese, crumbled feta, eggs, chopped dill and prepared spinach. Add the salt and mix well. Taste before adding more, since feta can be quite salty. The filling should be creamy but not watery. If your cottage cheese holds a lot of liquid, drain it before mixing.

Assemble the pie. Place the chilled puff pastry on the prepared baking pan. Spread the filling evenly over the pastry, leaving a border of about ¾ inch (2 cm) all around. Fold the edges slightly over the filling to create a rustic border. In a small bowl, whisk the egg yolk with the sour cream. Brush this mixture lightly over the exposed pastry edges for a golden, appetizing finish.

Bake until golden, rest and serve. Bake for 30 to 40 minutes, until the pastry is puffed and deeply golden and the filling is set in the center. If the edges brown too quickly, cover them loosely with aluminum foil for the rest of the baking time. Let the pie rest for about 10 minutes before slicing. This lets the filling firm up and gives you neat slices. Serve warm or at room temperature with a fresh green salad or sliced tomatoes.