Soak cashews in boiling water until totally soft, about 30-45 mins. Replace water if it cools down to speed up the process.
Place the bell pepper directly over heat on a gas stove, or under a broiler turning it often and keeping a close eye on it until it is totally charred and black and soft. Wrap in foil and let sit about 10 minutes or until cool enough to handle.
Drain and rinse the cashews and add to the pitcher of a high powered blender. Also add the garlic, shallot, salt and pepper and balsamic.
Peel the charred bits off the bell pepper and remove the stem and seeds inside. Add to the blender as well and blend until combined.
Scrape down the sides of the blender and with it running add in the olive oil in a slow stream until all combined and smooth. Season with more salt if necessary Keep covered in a container in the fridge until ready to use, and up to 2 days.