Trim fennel, cut vertically in half and retain the feathery fronds for garnish.
Slice fennel very thinly and add to a large bowl with the orange segments.
For the dressing:
Add the vinegar and seasoning to a bowl or jug and gradually whisk in the oil. Then stir in 3 tbsp of chopped mint.
Pour the dressing over the orange segments and sliced fennel. Toss and mix well to coat evenly. Arrange the salad on plates, sprinkle with the remaining mint and fennel leaves. For extra taste and crunch, scatter over the walnuts and olives.