1/3 cup packed light brown sugar
3 tablespoons Worcestershire sauce
12 ounces Coca-Cola, preferably Mexican (made with sugar, not corn syrup)
Heat the oil in a small saucepan over medium-high heat. Add the onions and cook, stirring, until soft and evenly browned at the edges, 8 to 10 minutes. Add the tomato paste and garlic and cook, stirring to combine both with the onions, for 30 seconds. Pour in the whiskey, bring to a boil and cook until reduced to a thick sauce, 2 to 3 minutes.
Add the ketchup, vinegar, brown sugar, Worcestershire, salt and soda; bring to a boil. Reduce the heat to maintain a steady simmer and cook, stirring occasionally, until reduced by about a third and thickened, 15 to 20 minutes.