Pre heat oven to 400 F and lightly grease a 9×13 casserole baking dish.

Chicken and vegetable filling:

Over medium heat, heat the oil and add the carrots, potatoes, onion, celery, mushrooms, salt, and pepper. Cook for 2-3 minutes, stirring frequently so none of the vegetable stick to the pan. Reduce heat to medium low and cover, stirring every few minutes. If vegetables start to stick, (particularly if you don’t use the mushrooms there may not be enough moisture in the pan) add a tablespoon of broth or water to help them steam better and not stick. Cook till veggies are mostly tender. They’ll cook a bit more in the oven so don’t cook them all the way or they’ll be mushy.

Remove the veggies to a bowl and return heat to medium. Melt the butter and then add the flour, whisking for 1-2 minutes. Slowly add the milk and the broth a little at a time till all incorporated and then bring to a simmer. Reduce heat to medium low and simmer till sauce thickens, about 6-8 minutes. Remove from heat and add in the frozen vegetables, cooked chicken, and veggies you set aside. Gently pour the mixture into your prepared baking dish and let cool a bit while you make the biscuits.

Biscuits-

In a small bowl whisk together the egg yolk and cream and set aside.

In another small bowl combine the buttermilk and sour cream till mixed together. Place in fridge till needed to keep cold.

In a medium mixing bowl combine the flour, baking powder, baking soda, and salt. Add the butter pieces and cut in the butter with a pastry blender till the butter pieces are about the size of small peas.  (or pulse all this together in a food processor).

Add the buttermilk and sour cream mixture and using a firm rubber scraper, press and mix the dough till it comes together. Add a teaspoon more at a time of buttermilk if you need a bit more moisture, but if you weighed your flour you really won’t need to.

Turn out on to a generously flour counter and press quickly into about a 7-inch x 10-inch rectangle. Fold the 2 short sides over to meet in the middle and quickly flatten down again to a 7×10 inch rectangle again. This creates flakey layers in the biscuits. Fold and flatten 2-3 more times.

When done folding, use a pastry brush to brush the top with the egg yolk and cream mixture. (you won’t use the entire mixture) Use a bench scraper or a sharp knife and cut the biscuits into squares. Quickly place the biscuits gently on top of the vegetable filling, careful not to submerge them. Place casserole baking dish in oven and bake for 35-40 minutes till biscuits are deep golden brown on top. Let rest for 10 minutes before serving.