1: Melt the butter in your cast-iron skillet pan (or medium saucepan) over medium heat, about 1 to 2 minutes.
2: Add the onions and garlic to the pan and cook over low to medium heat until softened, approximately 6 minutes.
3: Add the remaining ingredients and cook for 20 minutes over low heat.
4: Allow to cool completely, then transfer to the blender and blend until smooth.
If stored in an airtight container in the refrigerator, this sauce can last for 1 to 2 weeks.