Combine the ingredients for the creamy pesto sauce - basil pesto, mayo, sour cream, lemon juice and lemon zest in a small dish and whisk to combine. If needed, you can thin the sauce by adding water or milk to your desired consistency. Set sauce aside.
Pasta Salad
Cook the tortellini in a pot of boiling water, according to package instructions, until al dente. Drain and run under cool water to stop the cooking process.
Combine the cooked tortellini, chicken, tomatoes, mozzarella and creamy pesto sauce in a large bowl and toss together. Once coated, season with fresh basil, salt, pepper and red pepper flakes to taste.
Store pasta salad covered, in fridge until ready to enjoy.
Notes
For best results, refrain from adding the creamy pesto sauce to the pasta salad no more than 48 hours in advance.