In a large skillet over medium heat, melt the butter.
Add the onions and cook 3-4 minutes stirring constantly until translucent.
Sprinkle the flour, salt, pepper and nutmeg over the top of the onions and whisk together for 1-2 minutes to allow the rawness to cook out of the flour.
Add the whole milk and heavy cream and continue to whisk 1-2 minutes until thickened.
Add the rotisserie chicken and peas and carrots and stir to combine.
Spray a 9x13 casserole dish with cooking spray and transfer the mixture to the prepared dish.
Top the mixture evenly with all 8 biscuits.
In a small bowl, use a fork to beat the egg and brush onto the uncooked biscuits.
Bake uncovered for 15-18 minutes until the biscuits are golden. Serve immediately.
Notes
Store leftover chicken pot pie in the casserole dish covered or transfer it to an airtight container. It will keep for 1-2 days before the biscuits soften. For best results, you can remove the biscuits and store them separately.