Preheat oven to 350 degrees F. Melt the 1 Tbsp. of butter in a skillet set over medium heat. Add the onions and cook until tender, about 5 minutes; cool to room temperature. Whisk the flour, baking powder, sugar and salt together in a bowl. With your fingers, two forks or a pastry cutter, cut in the cold butter until the mixture is crumbly. Mix in cooked onions, cheese and parsley. Gently mix in buttermilk until a loose dough forms and then turn on to a lightly floured surface. Shape the dough into a ball and then flatten into a round loaf about 3- to 4-inches tall. Set on a parchment-lined baking tray. Cut a shallow X in the centre of the loaf. Brush the loaf with the 1 Tbsp. of buttermilk. Bake 45 to 50 minutes, or until cooked through. Serve warm or at room temperature.