In a medium sized sauté pan over medium, heat melt the coconut oil. Add the diced onion and cook while stirring for 1 – 2 minutes. Add the shrimp to the pan and sauté for several minutes, stirring occasionally, until they are pink, curled up and cooked all the way through. Turn the heat down to low and pour in the coconut milk along with the green curry paste.Be careful to mash up the green curry paste with the back of the cooking spoon so it will be absorbed into the milk. Pour in the fish sauce, if using. Once the coconut milk has slightly thickened and the paste is bound with the sauce, turn off the heat. Sprinkle the green onions and basil over the top of the green curry shrimp and serve with whole grain rice and a vegetable.