In a medium saucepan, heat oil. Cook onions over medium heat for 3-4 minutes, stirring often. Add garlic and cook for an additional 30 seconds until fragrant. Add remaining sauce ingredients (except butter) and simmer over medium heat for 3 minutes. Reduce heat to low and simmer another 5 minutes. Remove from heat, stir in butter until melted through. Let sauce cool for 10 minutes and place into a blender. Puree mixture and store in refrigerator until ready to use. Re-warm in microwave on high for 1 minute before using.