Make the dressing by whisking together the pesto with the mayo and lemon juice.
Heat oil in a skillet. Add pine nuts.
Toast them over medium-low heat in a small skillet for 3 to 4 minutes or until just lightly toasted.
Let cool.
In a big bowl toss together pasta with dressing.
Toss in onions, tomatoes, half the pine nuts and mozzarella cheese if using.
Refrigerate until ready to serve.
Before serving, sprinkle salad with remaining pine nuts and Parmesan Cheese and serve.
Notes
Pasta salads are best if served the same day they are made. If making the day ahead, do a dressing check. More dressing may be needed. Pasta tends to really soak it up! You can at a bit of olive oil, lemon juice and pesto if needed.