1. Preheat the oven to 190°C.
2. Melt the butter in a pan and add the flour. Cook for a minute, then add the milk whilst stirring vigorously. Stir all the time until it starts to thicken. Allow to come to a gentle simmer then remove from the heat and season with the garlic powder, salt, pepper and mustard. Add the cheese and stir until melted.
3. Combine the chicken, carrots and broccoli in a bowl and season it with the olive oil, lemon zest and fresh thyme.
4. Add the chicken mixture to the cheese sauce in the pan and mix well. Cool.
5. Roll out the puff pastry on a very well-floured surface so that it's large enough to cover the pie pan.
6. Place on top of mixture, pressing the edges and sealing. Cut a few slits on top of the crust, or poke holes with a fork to let steam escape.
7. Brush with egg wash and bake for 35-40 minutes, or until the crust is golden and the filling is bubbly. Serve immediately.