In a large skillet over medium heat, melt the butter. Add the oil and swirl to combine. Toss in the onions and 1/2 teaspoon of salt. Cook, stirring occasionally, until the onions start to soften, about 10 to 12 minutes. Reduce the heat to medium-low and continue cooking, stirring occasionally, until the onions turn golden and deeply caramelized, about 40 to 55 minutes more. Transfer to a medium bowl and let cool completely.