In a large skillet over medium heat, gently sweat the onions in the vegetable oil, for about 20 minutes, or deep golden brown – be careful not to burn. Set aside to cool to room temperature. Preheat oven to 450 degrees. Line a rimmed baking sheet with parchment or a silpat. In a large bowl, whisk together flour, baking powder, salt, and baking soda. Using two forks or a bench knife, cut the butter into the flour until butter pieces are about pea sized. Add the onions and toss in the dry ingredients. Add the buttermilk and stir just until most of the flour is absorbed by the buttermilk and the dough forms a coarse lump, about 1 minute. It will be a glunky mess. Dust a clean flat work surface with flour and gently fold the dough over itself until it comes together – do not overwork or stretch the dough. Press or gently roll the dough into a ¾-inch-thick square. Dip a biscuit cutter in flour and start cutting biscuits, dipping the cutter in flour between each biscuit. Press straight down to cut and lift straight up to remove; do not twist – this prevents the biscuit from rising Bake for about 10 minutes, or until golden brown. Let cool about 10 minutes, then serve with plenty of butter.