1 1/2 sticks cold unsalted butter, 1 stick cubed
1 large onion, finely chopped
1 tablespoon poppy seeds
Kosher salt and freshly ground pepper
1 cup coarsely shredded Gruyère cheese (3 ounces)
2 cups all-purpose flour, plus more for dusting
2 teaspoons baking powder
1/2 teaspoon baking soda
1 teaspoon salt
1 cup buttermilk