Place the sugar and vinegar into a non-reactive pan over a medium heat. Bring to the boil and cook for 3-4 minutes, or until the mixture is syrupy.
Add the orange zest and the peeled orange segments, the fennel, olive oil and olives, then remove from the heat and leave to cool.
Meanwhile, season the tilapia fillets with salt and freshly ground black pepper.
Heat the extra two teaspoons of oil in a frying pan over medium heat. Fry the tilapia fillets skin-side down for two minutes.
Turn the fillets over and add the butter. Fry for a further minute, or until the fish is just cooked through, basting regularly.
To finish the salad, place the tarragon and watercress into a bowl. Add the fennel and orange mixture and mix well.
To serve, place equal portions of the salad onto each plate and top each with a tilapia fillet.
