Preheat the oven to 180°C (fan bake).
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.
In the same pan, add a little more oil if needed. Add the diced onion, garlic, and mushrooms. Cook until softened and fragrant.
To 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.
Return the chicken to the pan. Add the milk, cream, Worcestershire sauce, paprika, thyme, and spinach. Stir gently over medium heat.
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.)
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.
Spoon the chicken and mushroom mixture into the filo-lined dish.
Crunch or fold the remaining filo sheets over the top, brush well with melted butter, and sprinkle with black sesame seeds if using.
Bake for 20–25 minutes or until the filo is golden, crisp, and beautifully flaky. Serve hot and enjoy!