Step 1Stir together the flour, sugar, baking powder, baking soda and salt. Cut in the butter until the mixture resembles coarse meal.
Step 2Dissolve the yeast in the warm water, then stir in the buttermilk. Add to the dry ingredients and stir until blended. Lightly knead on a floured surface. Roll out the dough into a 12-by-9-inch rectangle. Cut through the dough lengthwise, dividing into three parts, then crosswise into 4 parts to make 12 rectangular biscuits. Place on a baking sheet lined with parchment paper. Cover and let rise 1 hour.
Step 3Heat the oven to 450 degrees. Brush the tops of the biscuits with the cream. Sprinkle with the onion seeds, then pepper and salt. Bake until golden brown, 12 to 15 minutes. Remove to a wire rack to cool slightly.