Make the dressing. In a blender or small powerful food processor, combine half the avocado, the yogurt, salad onion, rice vinegar, olive oil, garlic and anchovies. Blitz until smooth, then add the tarragon, basil, parsley and watercress.
Add the lemon juice and a good grind of freshly cracked black pepper. Blend until smooth and vibrantly green. Add salt to taste, if liked, and adjust the texture with 2-3 tbsp water, if needed, for a drizzling consistency. Set aside.
For the salad, cut the top and bottom off the courgette and use a flat peeler or mandoline to peel long ribbons. Remove any large cumbersome stems from the watercress, and pull the leaves from the lettuce base, slicing any larger ones into manageable strips. Wash and dry all the leaves, making sure minimal water remains so the dressing clings well, and add to a large bowl with the courgette.
Slice the sugar snaps in half lengthways, discarding any woody stems, then add to the bowl. Add three-quarters of the dressing and toss together until everything is well coated.
Finely slice the radishes and avocado, including the reserved half from the dressing. Divide the salad between 2 bowls, then scatter over the radish rounds and arrange the avocado slices. Drizzle with the remaining dressing and serve.
