Preheat the oven to 180 C/350 F: You will notice a faint warmth spreading through the kitchen as the oven comes up to temperature, and preheating ensures even baking so the phyllo pastry crisps without steaming. This matters because an oven that is too cool will result in chewy pastry. A common mistake is to skip preheating, which makes the pastry absorb more moisture and not crisp properly.
Wash the spinach leaves that have been stripped from the stalk and roughly chop. Drain in a colander.: The spinach will be wet when you first handle it, so you will want to give it a good rinse and let it drain well; any excess water will steam during cooking and make the filling soggy. You should sense the cool, vegetal scent as you chop, and visually the leaves will collapse when cooked. One mistake to avoid is using un-drained spinach, which dilutes flavor and prevents the filling from binding well.
Heat a large nonstick frying pan and add a generous splash of olive oil. Fry the onion, leeks, and fennel until translucent over medium / low heat (about 5 minutes). Add the garlic and cook for a further minute or so.: As the aromatics cook, they will soften and release sweet, caramelized notes. You should hear a gentle sizzle that calms to an even simmer; the vegetables will become tender and glossy. Adding the garlic too early risks browning it and introducing bitterness, so time it carefully. A common misstep is cooking over too high heat, which causes rapid browning rather than gentle softening.
Add the chopped spinach in parts, wilting it as you go and until you can fit it all into the pan. add the dill, lemon zest, and juice and cook for about 5 – 8 minutes until soft and cooked through. Allow to cool slightly.: You will add the spinach in batches so each portion has a chance to wilt evenly, emitting steam and a fresh green aroma. The mixture will reduce dramatically in volume and release bright lemon fragrance when the zest and juice join it. Letting it cool slightly is important because piping hot filling can melt the cheeses unevenly and make assembly messy. Avoid overcooking; the leaves should remain tender and not turn muddy.
Once cooled, stir through the cream cheese, grated halloumi, Parmesan, chopped parsley, and mint. Adjust seasoning to your taste adding salt and black pepper as required.: As the cheeses melt gently into the warm spinach, the filling becomes creamy and cohesive, and you will feel it thicken as you fold. The aroma shifts to rich, savory notes, with herbal highlights from the parsley and mint . Taste to adjust salt, remembering that both halloumi and Parmesan can be quite salty. A typical error is under seasoning, which leaves the filling flat once baked.
Remove 4 sheets of phyllo from the roll quickly seal the remainder of the pack and either re-freeze or refrigerate until further use (I just refreeze).: The phyllo pastry is delicate and dries out quickly, so work swiftly and reseal what you are not using. You should feel the fragile, cool texture of the sheets and see how easily they tear if allowed to dry. Keep a lightly damp cloth over the sheets you are not using to prevent cracking. A common mistake is letting the sheets dry out, which causes them to crumble when you try to roll.
Roughly divide the mixture into 4 parts. Place the first sheet on a work surface with the thinner side at the bottom and spread one-quarter of the mixture along the bottom in a log shape. Lightly brush the whole top side of the sheet with butter. Roll the log of spinach up and place it in a lined baking pan.: When you spread the filling, you will feel the contrast between the crisp sheet and the soft filling, and the butter will make the phyllo pliable and glossy. Rolling carefully prevents tearing and ensures an even spiral. Press the seam lightly so the roll stays closed. Avoid overfilling, which can burst the pastry during baking.
Repeat with the remaining 3 sheets of phyllo, joining and twirling it into the pan. Allow a little space between the rolls although it should be snug. It does expand slightly when baked. Brush the tops generously with melted butter and sprinkle white and or black sesame seeds over the top.: As you place each roll, you will notice the pan filling out and the butter seeping into corners, which promotes golden crisping. The sesame seeds give a toasty aroma as they bake. Leave small gaps so heat circulates and the pastry can expand. One trap is packing the pan too tightly, which prevents even browning.
Bake for 40 minutes and until golden.: During baking the butter will brown and the phyllo will transform to a crisp, deep golden color; you will hear gentle crackles as the layers crisp. The filling should be set and not watery beneath the crust. If the top is browning too fast, tent lightly with foil to allow the interior to finish cooking without burning the phyllo. A common mistake is removing it too early, leaving a doughy texture.
Serve on its own or with tzatziki.: Let the rolls rest briefly so the filling firms up, then slice and serve. The warm filling will be creamy and fragrant, and a cool, tangy yogurt dip like tzatziki contrasts beautifully with the warm pie. Avoid serving piping hot straight from the oven as the filling can be very soft and may spill out when cut.