Heat a large skillet over medium high heat and add 2 tablespoons oil
Toss the chicken pieces in 1 tablespoon cornstarch and 1 tablespoon oil until well-combined
Sear the chicken pieces in the skillet until browned
Remove from the pan and set aside
If there are a lot of chicken bits stuck to your pan, deglaze with a ¼ cup of water
Then add the sliced onions to the skillet and cook until translucent
Add the curry paste and cook for 5 minutes
Add the vegetables and cook for 30 seconds
Add the coconut milk, chicken stock, lime leaves (if using), chili (if using), and the seared chicken pieces
Bring to a simmer and lower the heat
Cover and simmer for 10 minutes
After the curry has finished simmering, stir in the fish sauce, sugar, and basil
Serve with steamed rice