Heat 1 tablespoon of the oil in a 25 cm (10 inch) nonstick frying pan and swirl around. Beat the eggs in a large jug with the basil and plenty of pepper, then pour into the pan in a thin layer and cook for 1–2 minutes until golden and set. Remove and cut into thick strips.
Meanwhile, toss the feta and cherry tomatoes with the watercress in a serving bowl. Mix the remaining oil with the balsamic vinegar, pour over the salad and toss to coat.
Add the omelette strips, toss to mix and serve while still warm.