Step 1: Preheat the Oven
First, preheat your oven to 375°F (190°C).
Step 2: Prepare the Filling
Finely chop the spinach and place it in a bowl. Sauté the onion in olive oil until golden, then add it to the spinach. Stir in crumbled feta, season with salt and pepper, and mix well.
Step 3: Prepare the Sauce
In a separate bowl, mix the eggs with oil and milk until fully combined.
Step 5: Bake the Borek
Bake in a preheated oven for 45-60 minutes until golden and crispy. Let cool before serving.
Step 4: Assemble the Borek
Brush a 9×13-inch pan with olive oil. Layer 6 phyllo sheets, brushing each pair with sauce. Spread half the spinach mixture. Repeat with 6 more sheets and the rest of the spinach. Top with the remaining phyllo sheets, slice, and sprinkle with sesame and nigella seeds.