Place a skillet over medium heat and add 2 tbsp butter. Add the sliced onions and sprinkle with a little kosher salt. Stirring regularly, sauté the onions until tender and just beginning to brown. Remove from the heat and let cool to room temperature before using.
Preheat the oven to 400°F. Line a cast iron skillet, or baking sheet, with parchment paper and set aside. In a mixing bowl, combine the flour, baking powder, sugar, and kosher salt. Rub the remaining 2 tbsp butter into the dry ingredients until finely crumbled. Add three quarters of the sautéed onions to the mixing bowl and stir into the dry ingredients. Reserve the remaining onions to use later.
Add the stout to the dry ingredients and mix together with a wooden spoon. Switch to using your hands when the mixture becomes stiff and lightly knead until the dough comes together. Shape the dough into a ball and transfer to the prepared cast iron skillet. Use a sharp knife to score a cross on top. Scatter the remaining onions over the dough.
Bake for 15 minutes and then loosely tent the bread with a piece of foil. This will prevent the onions from burning before the bread is fully baked. Continue to bake for another 35 to 45 minutes until the bread is fully cooked and sounds hollow when tapped on the bottom. If desired, the foil can be removed during the last few minutes to brown the onions and bread. Transfer to a rack and cool.
Black Garlic Butter
Make the black garlic butter while the bread is baking. In the bowl of a food processor, combine the butter, black garlic, and kosher salt. Blend until the black garlic is well mixed into the butter - there may be some black flecks left but the garlic should be almost completely broken down. Scrape down the sides of the bowl as needed. Transfer to a serving bowl. Black garlic butter should be stored in the fridge.
This bread and butter are best enjoyed warm. Serve the black beer onion bread slathered with the black garlic butter and enjoy!