Stir together first 9 ingredients in a medium bowl until smooth. Stir in onion. Fold in pimiento and cheeses. Cover and chill 8 to 12 hours. Let stand at room temperature 30 minutes, and stir well before serving.
This recipe likely uses 1 2oz jar of pimientos- not 12oz. I added 1 4oz jar and drained off 1/2 the liquid. This was more than adequate for the recipe. You do not need to buy 3 4oz jars! (They don't even make a 12oz jar!)4 stars for taste, 2 for accuracy!