For poolish: In a medium bowl, stir together bread flour, ¾ cup plus 1 tablespoon (195 grams) water, all-purpose flour, and yeast until dry ingredients are completely moistened. Cover with plastic wrap, and let stand at room temperature for 14 to 16 hours.
For dough: In the bowl of a stand mixer fitted with the paddle attachment, place poolish. Add 1 cup (240 grams) water, oil, and yeast. Add flours and salt, and beat at low speed for 4 minutes.
Switch to the dough hook attachment. Beat at medium speed until dough gathers around dough hook and begins to pull away from sides of bowl, 8 to 10 minutes.
Oil a large bowl. Place dough in bowl, turning to grease top. Cover with plastic wrap, and let rise in a warm, draft-free place (75°F/24°C) for 1½ hours, folding dough in bowl after 30 minutes and again after 1 hour. (To fold, use a floured hand to reach under one side of dough, and pull gently over center; repeat so all 4 sides of dough are folded.)
Line a rimmed baking sheet with parchment paper.
Lightly flour surface with all-purpose flour; turn out dough onto prepared surface, and shape into a 12×6-inch rectangle. Using a bench scraper, cut dough into 8 portions (about 3-inch squares). Carefully transfer dough to prepared pan, lightly stretching dough as needed for even sides. Cover and let rise in a warm, draft-free place (75°F/24°C) until puffed and dough holds an indentation when pressed, 1 to 1½ hours.
Preheat oven to 425°F (220°C).
Bake until golden brown and an instant-read thermometer inserted in center registers 205°F (96°C), 12 to 15 minutes. Let cool completely on wire racks. Store in an airtight container for up 5 days. Tightly wrap in plastic wrap, and freeze for up to 3 months.