In a large sauce pot heat the olive oil over medium heat. Toss in the onions and sauté for 10 minutes or until translucent.
Turn the heat up to medium-high and continue to sauté for 1 minute more. Add in the apple cider vinegar and stir, while scraping the browned bits off the bottom of the pan.
Lower the heat to medium-low, add in the fennel and continue to cook for 20 minutes more or until the onion and fennel are caramelized.
Then add in the water, rosemary, bay leaf, cashews, salt and pepper and simmer for 5 minutes more.
Remove the soup from the heat, pull out the bay leaf and blend until smooth.
