Preheat oven to 450°.
To prepare roasted vegetables, combine first 6 ingredients in a large bowl. Place on a jelly-roll pan coated with cooking spray. Bake at 450° for 40 minutes, stirring once. Add mushrooms, tomatoes, and garlic; bake an additional 30 minutes or until vegetables are tender. Remove roasted vegetables from oven. Reduce oven temperature to 400°.
To prepare buttermilk pastry, lightly spoon flour into a dry measuring cup; level with a knife. Combine flour, baking powder, and salt in a small bowl. Add buttermilk, butter, and vinegar; toss with a fork until moist. Gently press mixture into a 6 x 4-inch rectangle on heavy-duty plastic wrap; cover with additional plastic wrap. Roll dough, still covered, into a 12 x 7-inch rectangle; freeze 10 minutes. Remove 1 sheet of plastic wrap; let stand 1 minute or until pliable.
Combine roasted vegetables, feta, and next 6 ingredients (feta through black pepper) in an 11 x 7-inch baking dish coated with cooking spray. Fit dough over filling. Remove top sheet of plastic wrap. Brush with egg white; sprinkle with sesame seeds. Cut 6 slits in top of dough to allow steam to escape.
Bake at 400° for 35 minutes or until the potpie is golden brown and bubbly around the edges. Let stand 10 minutes.