Bring a large pot of well-salted water to the boil and cook the tagliatelle according to the packet instructions until just before al dente.
While the pasta cooks, place one cup of the peas in a food processor with the rocket, garlic, chilli, mint, pine nuts, lemon juice, lemon zest, and olive oil. Pulse until you have a rough, textured pesto. Season well with salt and freshly ground black pepper and set aside.
When the pasta has about one minute left to cook, add the remaining cup of peas to the pasta water and cook for one minute. Drain, reserving about 60ml (¼ cup) of the pasta cooking water.
Return the pasta and peas to the pot with the reserved cooking water. Add the pea pesto and toss well to coat everything evenly. Tear the mozzarella into pieces and stir through the pasta.
Serve immediately in deep bowls, scattered with extra toasted pine nuts and a crack of black pepper.