To prepare the filling: In a large skillet, melt 2 tablespoons butter over medium-high heat, add in 2 cloves garlic and 1 large onion and cook until fragrant and tender, about five minutes. Add in 3 stalks celery and 3 large carrots and continue to cook for an additional 5 minutes.
Sprinkle 3 tablespoons all-purpose flour over the veggies, stir to coat, and cook for two minutes. Then add in 4 cups chicken or turkey broth, bring to a boil and add in 2 large potatoes and 1 cup peas. Reduce heat and simmer for 10-15 minutes, or until all veggies are tender.
Remove from heat, stir in 2 tablespoons fresh minced parsley, 3 cups shredded, cooked turkey, Salt and pepper. Set aside.
To prepare the biscuits: In a large bowl, combine 3 cups all-purpose flour, 1 tablespoon sugar, 4 teaspoons baking powder, 1 teaspoon salt, and 1 teaspoon baking soda until well-mixed.
Using a pastry blender, two forks, or your fingers, work ¾ cup chilled butter into the dry ingredients until the butter pieces are a touch smaller than a pea.
In small bowl, whisk together 1 ½ cup sweet potato puree and ½ cup buttermilk.
Gently stir in the buttermilk mixture to the dry ingredients, being careful not to over mix. The mixture will (and should!) be very sticky and liquidy—about the texture of a thick cake batter. If it isn't liquidy, add more buttermilk.
Heavily flour a work surface, and dump the dough onto the flour. Flour the top of the dough well, and then pat out until ½-inch thick. Fold the dough in half horizontally, then pat down again until ½-inch thick, adding more flour to cover sticky parts if necessary. Fold the dough in half vertically, then pat down again until ½-inch thick. Repeat this process 5-6 more times (this is creating the delicious, delectable layers that make the final biscuit so awesome).
Flour a circle biscuit cutter or a drinking glass, and press straight down to cut the biscuit. Do not twist the cutter! Twisting “seals” the sides of the biscuit and stops it from rising. Just push straight down and bring the cutter straight up.
To assemble the turkey pot pie: Pour the filling into an ungreased 9 x 13 baking dish, top with the biscuits (butting the biscuits right up next to each other). Bake in preheated oven for 30-40 minutes, or until the biscuits are golden brown and the filling is bubbly. Let cool for 10 minutes before serving.