Boil the Pasta: Bring a large pot of salted water to a boil. Add the penne pasta and cook for about 11 minutes, until al dente. Reserve 1 cup of pasta water, then drain.

Prepare the Pesto: Toast pine nuts in a skillet for 5 minutes. In a food processor, blend the toasted pine nuts, basil pesto, garlic, olive oil, Parmesan cheese, lemon juice, salt, and optional red pepper flakes until smooth.

Combine Ingredients: Return the drained pasta to the pot, add the pesto, cherry tomatoes, and mozzarella. Stir gently to combine.

Adjust the Consistency: Add reserved pasta water gradually until reaching desired sauce consistency. Taste and adjust seasoning if needed.

Serve and Garnish: Transfer to serving bowls and garnish with fresh basil leaves. Serve warm or chilled.