1. Cook the pasta in boiling water according to pack instructions, until just tender and al dente.
2. Meanwhile, place the garlic, pine nuts and capers in a food processor and blitz until finely chopped. Add the Parmigiano Reggiano, then all except a handful of the watercress. Blitz again until finely chopped. With the motor running, add the olive oil and finally the lemon juice. Season to taste.
3. Drain the cooked pasta, return it to the pan, then add the pesto, olives and tomatoes and toss together. Serve garnished with the reserved watercress.