Preheat oven to 220°C.
Heat oil in a large frypan and sauté onions until they start to brown in colour, add bacon, and continue sautéing for another 1-2 minutes.
Add in chicken pieces, along with dried thyme leaves and toss gently for 3-4 minutes until chicken pieces have cooked through.
Add the peas, carrots and mushrooms, stirring occasionally for another 2-3 minutes.
Pour in stock and milk, then stir in flour until well combined. Bring to the boil, then reduce heat to LOW and cook for another 5 minutes or until sauce thickens.
Transfer pie filling to a medium-sized, deep pie dish or baking tray. You can also use a few ramekins to make smaller individual servings. Place the frozen puff pastry on top of the baking tray or ramekins to enclose the filling and trim around the edges, leaving an excess of 1-2 inches.
Bake in the oven for 15 minutes or until pastry is puffed and golden.
