Add chopped onions and butter to a large skillet and cook over medium-medium low heat for about 20-30 minutes or until golden brown, soft and sweet. Season with salt and pepper and let cool.
Preheat oven to 400 degrees.
In a large bowl, whisk together the flour, baking powder, sugar, baking soda, and salt. Cut the cold butter cubes into the dry ingredients with a pastry cutter or fork until mixture becomes coarse crumbs.
Make a well in the center of the bowl and pour in the buttermilk and cooled caramelized onions, and stir until just combined. Dough will be sticky.
Flour a work surface, and turn dough onto the work surface. Create a dough ball and press into a rectangle, fold dough over itself about 4 times, that will create the flaky layers. Roll dough out about 3/4-1 inch thick. Cut with a 3-inch biscuit cutter. Repeat until all the dough is used, you should get about 12 biscuits.
Place biscuits on a parchment lined sheet pan, and brush with buttermilk. Bake for about 15 minutes or until golden brown. Serve warm. Best enjoyed the same day.