Prepare the Gravy:
1. In a small saucepan, whisk together the brown gravy mix and water.
2. Bring to a boil over medium heat, stirring constantly until thickened. Remove from heat and set aside.
Assemble the Casserole:
1. Preheat your oven to 375°F (190°C). Grease a 9×13 inch baking dish.
2. Spread the mashed potatoes evenly in the bottom of the prepared baking dish.
3. Layer the cooked popcorn chicken over the mashed potatoes.