Preheat a large skillet (10-12″) over medium-high heat. When the pan is hot but not smoking, add 1 tablespoon of the oil and swirl to coat the bottom of the pan.
Add the chicken breasts to the pan and cook until browned on both sides, 2-3 minutes per side. Then remove the chicken from the pan and set aside.
Add the remaining tablespoon of oil to the pan along with the onion, garlic, and ginger. Lower the heat to medium and cook, stirring often until the onion softens and is lightly browned, 4-5 minutes.
Next, add the optional chili, lime zest, and coconut milk. Bring the sauce to a boil, and using a wooden spoon, scrape up the browned bits from the bottom of the pan.
Lower the heat to maintain a simmer, and add the chicken breasts back to the pan. Cook for about 8-10 minutes, then turn the chicken breasts over and cook an additional 6-10 minutes, or until an instant-read thermometer registers 165ºF.
When the chicken is done, stir in the fish sauce and lime juice to the sauce, and season to taste with salt. Sprinkle the chopped fresh herbs over the top if using, and serve!