Cook pasta in salted water according to package directions. One minute before the pasta is finished, reserve 1 cup of cooking liquid. Then strain pasta through a colander and pour into a large bowl. Add tomatoes to the pasta and stir to combine.
Add the pesto on top of the pasta and slowly pour about 1/4 cup of the hot cooking liquid over the top, stirring quickly. The hot water will begin to melt the pesto into the pasta and allow it to distribute throughout the whole bowl. Continue to add the hot cooking liquid, 1-2 tablespoons at a time, as needed, until the pesto is well mixed throughout the bowl.
Add mozzarella pearls and olive oil and toss until all combined. Serve hot or cold. We like this best served cold after it has been refrigerated for 1-2 hours.
Add basil, cheese, pine nuts, garlic, lemon juice, salt and red pepper flakes to the bowl of the food processor and pulse about 10 times until everything is finely chopped.
With the food processor running on low, slowly drizzle in the olive oil and process until just combined. Your pesto should be a thick paste consistency but not too chunky. If needed, add more olive oil 1 tablespoon at a time, pulsing after addition, until the desired consistency is reached.