Preheat your oven to 375 degrees F. Coat a 9-inch x 13-inch baking dish with non-stick cooking spray. Set aside.
Place a large skillet over medium heat. Add the butter and let it melt. Add the onions, stirring occasionally until soft and translucent, 5-8 minutes. Add the garlic, stirring constantly until fragrant, about 30 seconds.
Add the frozen veggies to the skillet and cook until heated through, stirring occasionally, about 10 minutes. Add the half and half and cream of chicken soup and whisk it in until combined.
Add the turkey and pepper and stir it in until coated in the sauce. Heat until it just starts to bubble, taste, and add salt or more pepper if needed. Pour the filling into the prepared baking dish and set aside.
In a large bowl, stir together the flour, baking powder, sugar, salt, and baking soda. Add the butter to the bowl and cut it in with a pastry cutter or a fork. (You can use your fingers, but work quickly if you do!) The mixture should resemble coarse sand with bits of butter no larger than peas throughout. Add ¾ cup of buttermilk and stir it in until combined. There may be some loose flour in the bottom of the bowl. That’s okay.
Turn the biscuit mixture out onto a floured work surface, and knead it a few times until it comes together to form a dough. Roll out the dough into a rectangle ½-inch thick. Cut into 8 even pieces.
Place the biscuits right on top of the pot pie filling. Brush the tops with the remaining 1 tablespoon of buttermilk.
Bake for 20-25 minutes until the filling is bubbly and the biscuits are lightly golden brown. If the biscuits aren’t browned enough, put the broiler on high until they're browned to your liking.
Remove from the oven and let rest for 5-10 minutes. Sprinkle the top with some chopped parsley and serve warm.
Notes
You can substitute store-bought rotisserie chicken or pre-cooked chicken breast in place of the turkey.
To make with pie crust: Line two 9-inch pie plates with prepared pie crust and pour half the filling into each crust. Then top each pie with a second crust, crimp the edges closed, and cut a few steam vents in the top of each pie. Bake at 370 degrees F on the lowest oven rack for 40-45 minutes until the crust is golden brown.
To make with puff pastry: Cover the top of the baking dish with a sheet (or two) of thawed puff pastry, crimping it onto the edges of the dish. Brush the pastry with egg wash and then bake at 400 degrees F for 40-50 minutes until the puff pastry with golden brown.