Roughly cop the onion/leek and fennel. Chop the fennel fronds and set aside for garnish. Roughly chop the broccoli (including the stem).
In a large saucepan heat the oil, sauté the onion and fennel for 5-8 minutes, until soft. Add the garlic and potatoes, stir for a few seconds, then add the stock.
Bring the boil and simmer for 10 minutes, then add the broccoli and simmer for another 10 minutes, until the broccoli is cooked through.
Turn off the heat and add the herbs.
Using a hand-held blender blitz the soup until you get a smooth, silky texture. Taste and adjust the seasoning.
Serve with the chopped fennel fronds and a squeeze of lemon juice (or a drop of apple cider vinegar).