Melt the butter in a saucepan over medium heat. Add the onion and garlic and cook until translucent, about 3 minutes. Add the vinegar and boil until almost completely reduced and the mixture looks like wet sand, about 4 minutes. Whisk in both mustards and the jam or preserves. Simmer, whisking, until jam melts, about 1 minute. Remove the pan from the heat and stir in the bourbon and salt.