Bring a pot of salted water to boil. Toss the pasta and cook following the time on the package. Drain the pasta al dente, then cool it down immediately under cold water until the pasta is completely cold. (See notes)
Meanwhile, chop the tomatoes and mozzarella into small chunks and add them to a large bowl. Season with olive oil and salt.
Add the cooled pasta to the bowl and the pesto sauce. Season to taste, then mix well with a spoon and cover the bowl with wrap.
You can let it rest in the fridge for a few hours or serve it immediately. You can serve it with fresh basil leaves and a drizzle of chili oil or extra-virgin olive oil.
How to avoid overcooked sticky pasta: The secret to making the perfect pasta salad, is to drain the pasta al dente, then cool it down immediately under cold water until the pasta becomes completely cold. This way it will stop cooking and it will perfectly stay al dente for many many hours. Washing the pasta will also remove the starch so it will not be sticky.