Bring a saucepan of water to the boil and tip in the fregola. Simmer over a medium heat for 10 minutes until al dente.
In the meantime, add the orange and grapefruit slices to a bowl, then sprinkle over the lime zest. Mix in the watercress and basil, and leave to infuse.
Drain the water from the fregola and toss the fregola into the orange salad. Squeeze over the lemon juice and drizzle with the oil, then stir to combine.
Season with sea salt flakes and serve while the fregola is hot.