Directions

1. Combine 2 cups bread flour, 1 cup whole-wheat flour, 1 teaspoon yeast and ¾ teaspoon salt in the bowl of a stand mixer fitted with a dough hook attachment. Add ¾ cup water and ½ cup mashed banana; beat on low speed until a shaggy dough forms, about 1 minute. Cover the bowl with plastic wrap and let stand for 10 minutes. Beat dough on medium speed until a soft, tacky dough forms, 6 to 7 minutes. Add 1 tablespoon olive oil and continue kneading until incorporated. Cover and let stand for 15 minutes.
2. Meanwhile, in a small bowl, stir together ⅓ cup brown sugar, 3 tablespoons butter, 1 teaspoon cinnamon, ¼ cup chopped walnuts and the remaining ½ cup mashed banana until well combined.
3. Gently loosen the dough from the edges of the bowl using a flexible spatula; transfer to a clean, dry work surface. With damp hands, grasp the top edge (farthest away from you) and gently stretch the dough outward. Grasp the bottom edge (closest to you) and gently stretch toward you. Repeat the stretching gently with the left and right sides to form a roughly 9-inch square. Spoon half of the banana mixture (about ⅓ cup) onto the center of the square. Working with one corner at a time, fold each corner up and over the filling, gently pressing it down so it stays in place. Once all the corners are folded in, pinch together to seal. Place seam-side down, in a large bowl. Cover tightly with a plastic wrap and refrigerate until puffed, bubbly and doubled in size, at least 8 hours or up to 24 hours. Cover and refrigerate the remaining banana mixture.
4. One hour before baking, drizzle the remaining 1 tablespoon olive oil in a 10-inch cast-iron skillet. Gently flip the dough into the skillet. Using your fingertips, gently press and stretch the dough to evenly fill the pan, lifting one edge to let air bubbles underneath escape and repeating, moving around the dough, until there are no large air bubbles left underneath and the dough is evenly spread. Let stand at room temperature until puffy, jiggly and noticeably bubbly on the surface, about 1 hour. Meanwhile, adjust oven rack to lower third position and preheat to 450°F. Dimple the focaccia in about 1- to 1½ inch increments. Dollop the remaining banana mixture on top; arrange 4 walnut halves on top of the dough.
5. Bake until the focaccia is puffed, golden brown on both top and bottom, and crisp when you lift it with a thin spatula, 18 to 20 minutes. Sprinkle with ½ teaspoon flaky sea salt. Let cool in the pan for 5 minutes. Using a thin metal spatula, remove from the pan and transfer to a wire rack. Let cool for at least 15 minutes before cutting and serving. Serve warm or at room temperature.