Rather than cook tortellini according to packaging instructions, cook them 1-2 minutes less than what it says on the package. This way they will be cooked through, but sturdier. 

While the pasta is cooking, prepare your other ingredients. Dice the red onion finely, drain mozzarella balls, and slice tomatoes in half. 

In a large bowl, add pesto. Add drained pasta along with red onion and tomatoes and stir.

Wait five minutes to allow to cool slightly and then add in fresh mozzarella balls (this helps them stay intact and not melt).

Stir and enjoy warm or refrigerate for a cold pasta salad.