Cook bacon until crispy, then remove to a plate lined with paper towels to drain.
Retain about 2-3 tbs bacon fat in the skillet, and warm over medium. Once warm, add the onion and cook for 1-2 minutes. Add balsamic vinegar and continue cooking to reduce, about 2-3 minutes more. Remove from heat when done.
In the bowl of the slow cooker, mix sour cream and cream cheese. Add cheddar, mozzarella, onion, hot sauce, and ground mustard. Crumble or chop bacon and add it to the crock pot. Stir to ensure everything's mixed.
Cover and heat on high 1-2 hours until warmed through and cheese has melted. Serve with chips, crackers, baguette slices, or veggies.