In a large, shallow saucepan, combine the onion, garlic, and water. Cook over medium heat until the onion is translucent and softened, about 5 minutes.
2. Step
Add the ketchup, apple cider vinegar, honey, and mustard powder. Continue cooking for 20 minutes, until the sauce is thickened. Season with salt and pepper.
3. Step
Remove the pan from the heat. Using an immersion blender, puree the sauce until smooth. Let cool completely, about 25 minutes.
4. Step
Transfer to a clean airtight container and store in the refrigerator. The barbecue sauce will keep for up to 4 days.
