We like to use our Pizza Dough; however, you can use 1 pound ready-made pizza dough from the local pizzeria or supermarket. If you don't have a baking peel, use a rimless or overturned baking sheet to slide the flatbreads onto the baking stone. If you don't have a baking stone, you can use a preheated rimless or overturned baking sheet; however, the breads will be less crisp.

Adjust oven rack to lower-middle position, place baking stone on rack, and heat oven to 500 degrees. Let baking stone heat for at least 30 minutes or up to 1 hour.

Heat oil in 12-inch nonstick skillet over medium-low heat until shimmering. Stir in onions, sugar, and salt. Cover and cook, stirring occasionally, until onions are softened and have released their juices, about 10 minutes. Remove lid; increase heat to medium-high; and continue to cook, stirring often, until onions are deeply browned, 10 to 15 minutes.

Transfer dough to lightly floured counter, divide in half, and cover with greased plastic wrap. Working with 1 piece of dough at a time (keep other piece covered), press and roll dough into 14 by 8-inch oval. Transfer dough to parchment-lined baking peel and reshape as needed. Gently dimple surface of dough with your fingertips.

Brush dough liberally with oil and sprinkle with pepper. Scatter half of caramelized onions, ½ cup walnuts, and ½ cup blue cheese evenly over dough, leaving ½-inch border. Slide parchment paper and flatbread onto baking stone.

Bake until flatbread is deep golden brown, about 10 minutes, rotating flatbread halfway through baking. (Prepare second flatbread while first bakes.) Remove flatbread from oven by sliding parchment paper back onto baking peel. Transfer flatbread to cutting board, discarding parchment. Sprinkle with half of scallions, then slice and serve. Let stone reheat for 5 minutes before baking second flatbread.