Thaw chicken breasts in refrigerator overnight, then remove once thawed.
Pat chicken breasts dry with paper towels and season with kosher salt and black pepper.
Place chicken breasts in a large resealable bag and pour in the lemongrass marinade. Marinate for at least 6 hours or overnight in the refrigerator, flipping halfway through for even marination.
Remove the chicken breasts from the resealable bag and gently remove any excess marinade.
Preheat the oven to 375 degrees Fahrenheit.
Bring a large cast-iron pan to medium-high heat, then add grapeseed oil.
Sear one side of the chicken breasts for about 1 minute until nicely browned, being careful not to burn.
Place chicken breasts on a wire-racked lined sheet pan, with the seared side facing up. Cook for about 10-12 minutes until the internal temperature is 160 degrees Fahrenheit via a meat thermometer.
Remove the chicken breasts from the oven and transfer them to a cutting board. Let them rest for 5 minutes reaching an internal temperature of 165 degrees Fahrenheit before slicing.