HEAT a large skillet over medium high heat and add half your cooking oil. Once hot, add the chicken breasts to the pan, season with salt and pepper, and cook for 2 to 3 minutes on each side or until browned. Then remove the chicken from the pan and set it aside.
ADD the remaining oil to the pan. Once hot, add the onion, and garlic. Lower the heat to medium and cook until the onion softens and are lightly browned. This should take about 4-5 minutes.
ADD the chili (if using), lime zest, turmeric, and coconut milk and bring the sauce to a boil. Once boiling, lower the heat to maintain a simmer, and add the lime juice and fish sauce. Add your spinach (if using) and give everything a stir to help the spinach wilt. Add back the chicken breasts and cook for 8-10 minutes. Turn the chicken breasts over and cook an additional 8-10 minutes.
TASTE your sauce and season with salt, if needed. Sprinkle the chopped fresh cilantro and serve.