Preheat oven to 425 degrees F (220 degrees C). Line a baking sheet with parchment paper.
In a large mixing bowl, whisk together bread flour, salt, and baking powder until thoroughly combined. Beat in the butter, buttermilk, and confectioners' sugar to make a dough; gently mix in the onion and Cheddar cheese. Divide dough in half, and shape each half into a ball. Place the loaves onto the prepared baking sheet, and gently flatten to about 2 inches thick. Dust each loaf with flour.
Bake on a preheated oven until browned, about 30 minutes. Cool on racks for a few minutes; serve warm.