Melt 2 tablespoons butter in a Dutch oven; add half of onion, carrot, and celery. Add 1 teaspoon salt and pepper; saute 5 to 6 minutes or until tender. Add wine; cook 3 to 4 minutes or until reduced by half. Add broth; bring to a boil. Reduce heat, and simmer 15 minutes.
Stir together 1/2 cup flour and milk; add to wine mixture, and cook, stirring occasionally, 8 to 10 minutes or until thickened. Stir in turkey and peas. Pour into a 1 1/2-quart baking dish. Set aside.
Cut 1/4 cup butter into pieces.
Pulse butter pieces, remaining 3/4 cup flour, remaining 1/4 teaspoon salt, baking powder, and soda 4 to 6 times or until crumbly. Add remaining half of onion and sage; with machine running, gradually add buttermilk through food chute, processing until dough forms a ball. Turn out onto a heavily floured surface; knead 3 or 4 times. Roll into a 10-inch circle. Place over turkey mixture, pressing to edges of dish to seal. Melt remaining 2 tablespoons butter; brush over dough. Make 1/4-inch-deep slashes in top with a knife.
Bake pot pie at 425° for 30 minutes or until golden brown.