Preheat the oven to 425°F / 220°C. Toss 300g of chopped, trimmed spinach with ½ teaspoon of salt, leave for 10 minutes, then squeeze dry in a kitchen towel, a handful at a time.
Mix the spinach with 175g of crumbled feta, ½ cup of Greek yogurt, 2 finely sliced scallions, 1 egg, 2 tablespoons of chopped mint, 1 tablespoon of chopped dill, and 2 minced garlic cloves.
Stir in ½ teaspoon of ground nutmeg, ¼ teaspoon of black pepper, ¼ teaspoon of salt, ¼ teaspoon of cayenne pepper, ½ teaspoon of lemon zest, and ½ tablespoon of lemon juice.
Trim 16 filo sheets to 12½ × 10in / 32 × 25cm and melt 120g of butter. On a parchment-lined baking sheet, layer half the filo, brushing each sheet with butter.
Spread the filling over, leaving a 1in / 2.5cm border. Top with the remaining filo, brushing each sheet with butter and sprinkling the first five with 60g of finely grated kefalotyri.
Brush the top with butter and sprinkle with ½ teaspoon of white sesame seeds and ½ teaspoon of black sesame seeds. Bake for 25 minutes, until golden brown.
