Phyllo Turkey Pot Pie

Name: Phyllo Turkey Pot Pie
Ingredients: 14 Tbs. (1 3/4 sticks) (7 oz./220 g) unsalted butter; 1 cup (5 oz./155 g) plus 2 Tbs. all-purpose flour; 1/3 cup (3 fl. oz./80 ml) Madeira; 2 Tbs. chicken demi-glace; 7 cups (56 fl. oz./1.75 l) chicken stock; 1 large yellow onion, diced; 4 celery stalks, sliced 1/8 inch (3 mm) thick; 1 lb. (500 g) cremini mushrooms, thinly sliced; 1 Tbs. chopped fresh thyme; 2 tsp. chopped fresh tarragon; 1 bay leaf; 8 oz. (250 g) small red-skinned potatoes, finely diced; 8 cups cubed cooked turkey (about 3 lb./1.5 kg total); 1 bag (1 lb./500 g) frozen pearl onions; Kosher salt and freshly ground pepper