Make the Biscuits:
In a large bowl, whisk together the flour, baking powder, sugar , and salt. While stirring with a wooden spoon, slowly pour the cream into the center of the bowl, and continue mixing until a dough just forms. Transfer the dough to a lightly floured work surface and roll into a 3/4-inch thick rectangle. Cut the dough into 6 large, equal-sized squares and transfer them to a parchment paper-lined baking sheet. Refrigerate until ready to use. (The biscuit may be prepared up to day in advance.)
Make the Filling:
Preheat the oven to 350 degrees F.
Place the turkey breast in large roasting pan, cover with the chicken stock, and season with salt and pepper, to taste. Roast until just cooked through and an instant-read thermometer inserted in the breast reads 165 degrees F. Transfer the turkey breast to plate and let cool. Pull the turkey into small pieces and set aside.
Transfer the chicken stock to a saucepan , bring to a boil, and reduce to 1 1/2 cups.
Melt 2 tablespoons of the butter in a small saucepan. In another roasting pan, toss together the carrot, rutabaga, parsnip , and turnip with the butter and season with salt and pepper, to taste. Roast until browned and tender, about 35 to 40 minutes.
Heat 2 tablespoons of the butter in a large skillet over medium-high heat. Add the onion and cook, stirring, until translucent, about 6 to 8 minutes. Add the mushrooms and cook, stirring occasionally, until browned, about 10 minutes. Set aside.
Meanwhile, heat the remaining 2 tablespoons butter in a saucepan over medium heat. Add the flour and cook, stirring, for 5 minutes. Add the reduced chicken stock , bring to boil, lower the heat and simmer for 15 minutes.
While the sauce is simmering, bake the biscuits until golden brown, about 15 minutes.
Stir the cream into the sauce and return to a boil. Stir in the pulled turkey , roasted vegetables, onion-mushroom mixture, thyme , and parsley. Season the filling with salt and pepper, to taste. (The filling may be prepared up to 3 days in advance, and reheated before serving.)
Divide the hot filling among 6 warmed 12-inch wide bowls. Place a biscuit in the center of each and serve immediately.
