Slice your onion thinly. Mince garlic, ginger, and chilis (if using) and place in a small bowl. Add the Indian curry paste and lemon juice and mix well. This will be the spice you use to coat the fish!
Now coat both sides of the fish with the spice paste.
Heat your nonstick pan or cast iron skillet to medium high heat. Add the olive oil (or vegetable oil), and once hot add the sliced onion. Fry the onion for around 6 minutes, or until the onion is soft and browned. Move the onion to the side of the pan.
Lower the heat to medium. Add the fish (in a single layer) which has been coated with the spice paste to the center of the pan. Fry each side for 2 minutes. Then add the coconut milk.
Allow to simmer at just under a medium heat for approximately 10 minutes. Add salt to taste, and then garnish with coriander.
