In a medium saucepan, melt butter over medium heat, add onion and cook until soft and translucent, about 5 minutes. Add garlic and red pepper flakes and cook for 30 seconds.
Add the remaining ingredients. Bring to a simmer, cook 10 minutes over medium-low heat.
Optional, puree with an immersion blender or high-speed blender to smooth out the onion.
Notes
Leftover BBQ sauce can be stored in the fridge for several weeks in an airtight container or jar.
You can also freeze BBQ sauce for up to 6 months. Be sure to leave headroom in the jar.