Preheat the oven to 425 F. Line a baking sheet with a piece of parchment paper.
Cut the cauliflower into florets if not already done. Chop the fennel bulb into chunks. Peel and chop the onion. Place these on the baking sheet, drizzle with the oil and sea salt. Toss everything to coat and then spread out evenly on the sheet. Bake for 25 minutes.
When the mixture is done roasting, carefully pour it into a large mixing bowl or stand blender. (You may have to do it in batches if using a stand blender) Add the chicken broth and remaining salt and then blend with either an immersion stick blender or with the stand blender. (If doing it in batches, pour in the corresponding amount of stock, so if you added half of the mixture, add half of the stock)
If the soup is going to be served immediately you may want to warm it up a bit on a pan on the stove.
Pour into individual serving bowls, then scoop about 2 tbsp of pesto on top of each bowl and swirl gently with a spoon. Garnish if desired with minced fennel fronds and basil. Serve!