Trim the top of the fennel and save the leafy greens for the dressing. Peel the outside of the fennel bulb if it appears to be tough or is discoloured. Core the bulb and then halve and thinly slice.
Place the vinegar, orange juice, oil, chopped fennel leaves, salt and pepper in a large bowl and whisk to combine. Add the sliced fennel, tomatoes and oranges and gently toss to combine. Let the flavours meld for 20 minutes, then gently toss again before serving.