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 up the heat 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 15 minutes more. The vegetables will start to caramelize into a rich brown.
Then add in the water, rosemary, bay leaf, cashews and season with a heavy hand of salt and pepper. Simmer for 5 minutes more.
Remove the soup from the heat, and pull out the bay leaf. Blend until smooth and serve warm.