Preheat your oven to 400°F (200°C) and ensure your puff pastry is thawed but still cold.
Clean the leeks thoroughly and slice the white and light green parts into thin rounds.
Heat the olive oil in a medium non-stick skillet over medium heat; add the leeks and sauté for 4-5 minutes until softened.
Add the diced chicken breast to the skillet, seasoning with sea salt, black pepper, and dried thyme.
Cook the chicken for about 5-6 minutes until browned on all sides and mostly cooked through.
Pour in the chicken broth and add the frozen peas, allowing the mixture to simmer for 2-3 minutes until the liquid reduces by half.
Remove the skillet from the heat and stir in the Greek yogurt until a smooth, creamy sauce forms around the chicken.
Transfer the chicken mixture into a single-serve oven-safe ramekin or small baking dish.
Roll out the puff pastry slightly and place it over the top of the ramekin, pressing the edges to seal.
Lightly brush the top of the pastry with water and cut a small slit in the center to allow steam to escape.
Bake for 15-20 minutes until the pastry has risen and turned a deep golden brown.