Preheat your oven to 400°F (200°C).
Dice the chicken breast into small, uniform cubes and season with garlic powder, sea salt, and black pepper.
Heat the olive oil in a skillet over medium heat and sauté the chicken until it is cooked through and lightly browned.
Add the diced onions and sliced mushrooms to the skillet, cooking for 4-5 minutes until the vegetables are soft and fragrant.
Stir in the fresh spinach until it just begins to wilt, then remove the skillet from the heat.
Fold in the Greek yogurt until the chicken and vegetables are thoroughly coated in a creamy sauce.
Transfer the mixture into a small oven-safe baking dish or ramekin.
Drape the puff pastry over the top of the dish, pressing the edges firmly to create a seal.
Whisk the half egg and brush a thin layer over the pastry to ensure a glossy, golden finish.
Bake for 15-18 minutes until the pastry has puffed up and turned a beautiful golden brown.