Melt 2 tablespoons of the unsalted butter in a 12-inch cast iron or other broiler-safe, heavy bottomed skillet over medium heat. Add the onions, thyme, 1/2 teaspoon kosher salt, and 1/2 teaspoon black pepper. Cook, stirring frequently with a wooden spoon and reducing the heat if the onions start to brown too quickly, until the onions have reduced by over half, are very soft, and deep golden-brown, 25 to 30 minutes.