How to Make It
Melt butter in a large saucepan over medium heat. Add onion and garlic; sauté 5 to 6 minutes or until tender. Add remaining ingredients. Bring to a boil; reduce heat, and simmer 30 minutes, stirring often. Remove and discard lemon slices.
Note: Sauce can be made ahead and kept covered in refrigerator for up to 2 weeks.