Prep the chicken. If using leftovers or rotisserie, remove skin and bones, then chop or shred into bite-size pieces.
You want roughly 3 cups. Let it come to room temperature for the best texture.
Make the dressing base. In a large bowl, whisk pesto with olive oil and lemon juice until glossy and spoonable. Taste and adjust: add more lemon for brightness or a splash more oil to make it silkier.
Add aromatics. Stir in the minced red onion or shallot.
This gives a sharp edge that balances the rich pesto.
Fold in the chicken. Toss to coat every piece with the pesto dressing. Season with salt and pepper. Let it sit for 5 minutes so the flavors settle in.
Add mozzarella and tomatoes. Gently fold in the mozzarella and halved cherry tomatoes.
Be careful not to crush the tomatoes.
Add herbs and heat. Sprinkle in the torn basil and a pinch of red pepper flakes if you like a little kick.
Finish and serve. Taste again and adjust. Serve over a bed of baby spinach or arugula, or mix the greens directly into the salad for a heartier bowl. Top with toasted nuts if using.