1. Weigh or lightly spoon flour into dry measuring cups; level with a knife. Combine flour, 1/2 teaspoon salt, and baking powder in a food processor; pulse 2 times to combine. Combine 1/3 cup oil and 1/4 cup water in a small bowl. With processor on, slowly add oil mixture through food chute; process until dough is crumbly. Turn dough out onto a lightly floured surface. Knead 1 minute; add additional flour, if necessary, to prevent dough from sticking. Gently press dough into a 5-inch disk; wrap in plastic wrap, and chill 30 minutes.
2. Remove stems from chard leaves; chop stems to equal 1 cup. Chop leaves to equal 4 cups. Heat a large skillet over medium-high heat. Add remaining 1 tablespoon oil to pan; swirl to coat. Add chard stems and onion to pan; sauté 1 1/2 minutes. Add chard leaves to pan; sauté 2 1/2 minutes. Stir in remaining 1/4 teaspoon salt, potato, and thyme. Remove from heat; cool.
3. Preheat oven to 375°.
4. Unwrap dough, and roll into a 14-inch circle on a floured surface. Place dough on a baking sheet lined with parchment paper. Spread chard mixture evenly over dough, leaving a 2-inch border. Fold edges of dough toward center, pressing gently to seal (dough will only partially cover chard). Combine 1 teaspoon water and egg white in a small bowl, stirring with a whisk. Brush dough edges with egg white mixture. Sprinkle pepper and cheese over chard mixture. Bake at 375° for 40 minutes or until browned. Let stand 5 minutes; cut into 12 wedges.