Heat the 2 tbsp of olive oil in a large, wide pan over medium heat. Add the finely chopped onion and cook for 6–8 minutes, stirring occasionally, until completely softened and showing light golden colour at the edges. The full cooking time for the onion is not negotiable in a börek application. Raw or under-softened onion placed inside the pastry continues cooking during the oven's 40–50 minutes — releasing moisture progressively into the surrounding phyllo layers and producing the soggy, structurally compromised base that is börek's most common failure mode. A fully softened, lightly golden onion has already released and cooked off its moisture on the stovetop; it contributes its sweet, aromatic character to the filling without contributing any further liquid in the oven. Add the 3 minced garlic cloves and cook for 30 seconds, stirring constantly, until fragrant but not browned. Add the 700g of baby spinach in 3–4 batches — adding all at once overloads the pan and drops the temperature, steaming the spinach in its own moisture rather than wilting it properly. Add the first batch and fold with tongs until wilted before adding the next. Continue until all the spinach has been added and is fully wilted and collapsed. Cook for a further 2–3 minutes, stirring and folding, until most of the visible moisture in the pan has evaporated. The pan should not have any pooling liquid.

Combine the cooled, pressed spinach mixture with 250–300g of crumbled feta in a large bowl. Season generously with freshly cracked black pepper. Taste before adding any salt — feta carries significant inherent salinity and the filling is often sufficiently seasoned from the cheese alone. The filling should taste assertively flavoured and well-seasoned — it will be distributed across a large quantity of phyllo, which is neutral, and its flavour will be diluted by the layers.

In a medium bowl, whisk together the 100ml of olive oil, 200ml of whole milk, 200g of whole-milk yogurt, 1 egg, optional ¼ tsp of salt, and black pepper until completely smooth and fully emulsified — no visible oil separation. This mixture is the element that differentiates this börek from a simple layered phyllo pastry and produces the specific textural contrast that makes it compelling. Applied lightly between each phyllo sheet, it hydrates each sheet just enough to prevent it from baking dry and brittle throughout — the outermost layers that receive the most oven air exposure dry and crisp; the interior layers that are shielded by surrounding layers remain soft, yielding, and slightly custardy from the egg and yogurt's proteins. The olive oil in the mixture contributes richness to each layer; the milk and yogurt provide the moisture and the slight tang; the egg provides the structure that allows the interior to set into the custardy texture rather than remaining wet.

Preheat the oven to 180°C fan or 190°C conventional. Lightly brush a 23×33cm (9×13 inch) baking tray with olive oil or line with parchment paper. Remove the phyllo dough from its packaging and lay the sheets flat, covered with a slightly damp cloth — phyllo dries and becomes brittle within minutes of exposure to air. Work with one sheet at a time, keeping the remaining sheets covered. Place the first phyllo sheet in the tray — it will likely be larger than the tray and can be folded at the edges or draped over the sides. Spoon or brush a light, even coating of the milk-yogurt mixture over the entire surface. Light is the operative word — the sheet should show visible dampness but not be saturated or pooling with liquid. A heavily drenched phyllo sheet bakes dense and bread-like rather than producing the delicate, distinct layers of a properly assembled börek. Continue laying phyllo sheets and lightly coating with the milk-yogurt mixture until approximately half the total sheets have been layered. Do not count sheets rigidly — work by eye, ensuring even distribution and adjusting the mixture quantity to maintain the light coating through all the bottom layers. You may not need the entire milk-yogurt mixture and it is better to have leftover mixture than to over-apply.

Spread the spinach and feta filling evenly across the surface of the top bottom-layer sheet — distributing it all the way to the edges of the tray so every portion receives filling through to the corners. An uneven filling layer produces some portions that are overwhelmingly cheesy and some that are predominantly pastry. Continue layering the remaining phyllo sheets on top of the filling, applying the same light coating of milk-yogurt mixture between each sheet. Fold in any excess phyllo at the edges or trim with kitchen scissors for clean square edges — the trimmed edges can be scattered on top of the final sheet for additional layered texture.

Whisk the 1 egg yolk with 1 tbsp of water until completely smooth. Brush evenly across the entire top surface of the final phyllo sheet — reaching all the way to the edges. Scatter sesame seeds and nigella seeds generously across the egg-washed surface. Nigella seeds — small, angular, black, with a specific slightly bitter, slightly onion-adjacent aromatic character — are the seed that specifically identifies the börek as Turkish in character; their flavour and visual contrast against the golden sesame makes the finished top surface immediately recognisable. Together the two seeds toast against the egg wash during baking into a fragrant, slightly crunchy surface layer. Using a sharp knife, cut the börek into 16 equal squares before placing it in the oven. Apply firm, decisive downward cuts rather than sawing — sawing drags the top phyllo layers and disrupts the even surface. This pre-baking cut is the technique that makes clean, intact squares possible after baking. A knife drawn through baked, crisped phyllo shatters the top layers regardless of sharpness; the same cuts made through unbaked, pliable phyllo produce clean divisions that hold through baking.

Place in the preheated oven and bake for 40–50 minutes until the top is deeply golden brown across the entire surface — a pale gold is insufficiently baked and will not have the crunch of a properly coloured börek — and a knife inserted through the centre feels resistance-free, indicating the layers have fully set. If the top surface shows significant browning before 35 minutes, loosely tent with foil for the remaining baking time to prevent burning while the interior continues setting. Remove from the oven and allow to rest in the tray for a minimum of 15–20 minutes before serving. Fresh-from-the-oven börek is structurally fragile — the filling is still liquid-adjacent from its internal temperature and the pastry layers need the resting period to fully set. Attempting to lift portions before resting produces pieces that fall apart or leave their bottom layers behind. After the full rest, use a spatula to lift each pre-cut square cleanly. Serve warm or at room temperature.