Preheat the oven to 425°. Butter a 9-by-4 1/2-inch metal loaf pan. In a large skillet, melt the 1/2 stick of uncubed butter; pour 2 tablespoons of the melted butter into a small bowl and reserve. Add the chopped onion to the skillet and cook over moderate heat, stirring occasionally, until it is softened, about 8 minutes. Stir in the poppy seeds and season with salt and pepper. Scrape the onion mixture onto a plate and refrigerate for 5 minutes, until cooled slightly. Stir in the Gruyère.
Meanwhile, in a food processor, pulse the flour with the baking powder, baking soda and salt. Add the cubed butter and pulse until it is the size of small peas. Add the buttermilk and pulse 5 or 6 times, just until a soft dough forms.
Turn the dough out onto a well-floured work surface and knead 2 or 3 times. Pat or roll the dough into a 2-by-24-inch rectangle. Spread the onion mixture on top. Cut the dough crosswise into 10 pieces. Stack 9 pieces onion side up, then top with the final piece, onion-side down. Carefully lay the stack in the prepared loaf pan and brush with the reserved butter.
Bake the loaf in the center of the oven for about 30 minutes, until it is golden and risen. Let the bread cool for at least 15 minutes before unmolding and serving.