Instructions:
- Melt butter in a medium heavy-bottomed saucepan over medium heat. Add onion and sauté gently until translucent and soft, about 5-7 minutes. Add garlic and ginger (if using); cook for 1 minute until fragrant.
- Stir in the ground mustard powder and allspice. Cook for 30 seconds until fragrant to bloom the spices.
- Pour in the broth, cider vinegar, ketchup, Worcestershire sauce, and molasses/treacle. Stir well to combine all ingredients.
- Bring the mixture to a gentle simmer. Reduce the heat to low, allowing it to bubble very gently. Simmer uncovered for 30–45 minutes, stirring occasionally, until the sauce has visibly reduced by about one-third.
- Remove from heat. Stir in the salt, pepper, and cayenne. Taste and adjust seasoning—add more sugar for sweetness, vinegar for tang, or Worcestershire for depth as desired.
- Optional: If a perfectly smooth sauce is desired, pour the mixture through a fine-mesh sieve into a clean bowl, pressing down on the solids. If you prefer texture, skip this step.
- If the sauce is not thick enough, return it to the pan. Whisk in the cornstarch slurry and bring back to a gentle simmer for 1 minute until it coats the back of a spoon nicely.
- Allow the sauce to cool completely before transferring it to an airtight jar for storage. The flavour improves significantly overnight.