• Preheat oven to 375°.

Prepare chicken & gravy filling:

• Melt butter in a large cast iron skillet over medium heat.
• Sauté diced onion until soft and translucent.
• Stir in ⅓ cup flour and stir until flour starts to brown just ever so slightly.
• Add 3 cups milk and cook until sauce starts to thicken, about 2-3 minutes.
• Add shredded chicken, mushrooms, carrots and peas, salt and pepper. Stir to combine.
• Remove from heat and set aside while you prepare the biscuits.

Prepare biscuits:

• In a large bowl, combine 4 cups flour, 4 teaspoons baking powder, 1 teaspoon baking soda and salt.
• Using a pastry cutter, cut cold butter into the flour mixture. Combine until butter resembles coarse sand.
• Stir in buttermilk to combine with the dough.
• Flour work surface, using your hands form dough into a large rectangle, about 1 inch thick.
• Cut out biscuits and place them in an even layer on top of the chicken gravy mixture in the cast iron skillet.

Bake casserole:

• Brush biscuits with beaten egg.
• Bake at 375° for 25-30 minutes until chicken gravy is hot and bubbly and the biscuits are lightly browned.
• Top biscuits with shredded cheese and continue to bake for another 5 minutes until cheese is melted and stew is hot and bubbly.