To make the dressing, whisk all the ingredients together.
Fill a pan with water, add a pinch of salt and bring to the boil. Blanch the leeks until they are very soft and sweet – this should take a couple of minutes – then remove and set aside to drain. When the water is boiling again, blanch the fennel for 15 seconds, then remove and drain. Blanch the turnips for around 4 minutes, then remove and drain. Blanch the carrots until al dente. Finally, blanch the asparagus for 30–40 seconds and drain.
Put a tablespoon of olive oil into a pan over a medium heat and fry the courgettes for around a minute, until al dente with slightly caramelized skin.
Mix all the cooled vegetables together (including the uncooked radishes) with the leaves from the turnips and radishes if you have any, and add the dressing. Serve cold.