In a large pot melt the butter on medium-low heat and cook onions with a pinch of salt until soft and translucent (about 5-10 minutes).
Add the stock and peeled and chopped potato.To poach the eggs, bring a separate pot of water to the simmer along with a generous amount of salt and the vinegar.
Crack the eggs, one at a time into a small teacup and gently drop the egg into the water. Once cooked, remove with a slotted spoon and set aside.
To make the olive garnish, mix together the olive tapenade and some olive oil to get a looser consistency. For the goats cheese garnish, mix the goats cheese with the lemon juice and just enough water to loosen it up.
Once the potatoes have softened, roughly chop zucchini and broccoli (stem and all) and place in the pot. Top with some extra water until just covered.
Simmer for 3 minutes with the lid on, until just cooked through. Remove from heat and add basil, mint and watercress.
Blend in batches using a blender, making sure you cover the blender with a tea towel, and re-stir the batches to distribute the flavours evenly.
To serve, pour soup into large bowls, top with a poached egg, some olive tapenade and goats cheese.