Preheat the oven to 180°C (fan bake).
Cook the chicken:
Heat the oil in a large pan over medium heat. Add the diced chicken and cook until browned and mostly cooked through. Remove and set aside.
Sauté the aromatics:
In the same pan, add a little more oil if needed. Add the diced onion, garlic, and mushrooms. Cook until softened and fragrant.
Make the roux:
In a small bowl, use a fork to mash together the soft butter and plain flour until it forms a paste. Set aside.
Build the sauce:
Return the chicken to the pan. Add the milk, cream, Worcestershire sauce, paprika, thyme, and spinach. Stir gently over medium heat.
Thicken the filling:
Add the roux paste a little at a time, stirring constantly until the sauce thickens into a creamy consistency. (You may not need all of it — stop when you’re happy with the texture.)
Assemble the pie:
Lightly grease a pie dish or baking dish.
Lay sheets of filo pastry into the dish one at a time, brushing each layer lightly with melted butter. Allow some overhang for a rustic folded edge.
Add the filling:
Spoon the chicken and mushroom mixture into the filo-lined dish.
Top with filo:
Crunch or fold the remaining filo sheets over the top, brush well with melted butter, and sprinkle with black sesame seeds if using.
Bake:
Bake for 20–25 minutes or until the filo is golden, crisp, and beautifully flaky.
Serve hot and enjoy!