Preparation Whisk ginger ale, yellow mustard, and Dijon mustard in a medium saucepan to combine. Bring to a simmer over medium heat; reduce heat to low and gently simmer 3 minutes. Whisk in ginger, garlic, onion, honey, sugar, salt, vinegar, black pepper, and red pepper flakes and cook, stirring occasionally, until flavors have come together and sauce is very fragrant, 15–20 minutes. Let cool before using.Do ahead: Sauce can be made 2 days ahead. Transfer to an airtight container; cover and chill.