Set the oven to 350 degrees F and coat a 9-inch pie dish with butter or non-stick spray. Then, line the pie dish with one of the pie crusts.

Melt the butter in a large skillet on medium-high heat. Add the onion once the butter has melted and saute for 2 minutes until the onion is beginning to soften. Then, add the garlic and cook for another minute.

Whisk in the flour and cook for a minute. Slowly stir in the milk and cream cheese. Cook further until the cream cheese has melted and the mixture has thickened, stirring often. Next, season with salt, pepper, and ground mustard.

Turn the heat off and whisk in 1 1/2 c. of the cheddar cheese until melted. Add the broccoli and chicken. Stir well.

Into the prepared pie shell, pour the mixture, and cover with the rest of the cheese. On top, place the second pie crust, folding and crimping the edges to seal the crust. Then, slice 4 slits in the middle of the pie.

Combine the egg with 1 tbsp water, then brush this over the crust.

Place in the preheated oven and bake for about 40 to 45 minutes or until golden brown.

Remove from the oven when done, and allow the pot pie to rest for about 10 minutes before serving.