Melt 1 tablespoon butter in a large saucepan over medium heat; cook and stir onion in hot butter until starting to caramelize, 8 to 10 minutes. Add ginger to onion; cook and stir until fragrant and caramelized, 2 to 5 minutes more.
Stir brown sugar, cornstarch, cinnamon, star anise, mustard, cloves, salt, and pepper into onion mixture; cook until brown sugar and cornstarch dissolve. Slowly mix vinegar into brown sugar mixture. Add apple cider, raisins and bourbon; bring to a boil, stirring frequently, and cook until sauce is reduced by half, 15 to 25 minutes. Remove from heat.
Whisk 1 pound butter into sauce, 1 tablespoon at a time, until butter melts and sauce is glossy and thick.