1. With the chicken in a Ziploc bag, flatten chicken breasts with either a mallet or by pressing down with your hands, flattening the thickest part of the chicken as much as you can. Add 1/4 cup of pesto to the chicken in the bag and marinate in the fridge for a couple of hours before grilling.
2. About 20 minutes before grilling, remove the chicken from the fridge. Preheat the grill to medium-high. Place the chicken breasts on the grill and cook for 5-6 minutes per side, cooking until the chicken reads a temperature of 165 F. Remove from the grill, and using a pastry brush, brush the remaining pesto over the cooked chicken and serve immediately!

Leftover chicken can be kept in the fridge in an airtight container, for 3-4 days or up to 2 months in the freezer.