Rinse the fish fillets and cut them into bite-sized pieces. Set aside.
In a large pot or skillet, heat a small amount of oil over medium heat.
Add the finely chopped onion and sauté until translucent, about 5 minutes.
Stir in the minced garlic and fresh ginger, cooking for another minute.
Add the chopped tomatoes, green bell pepper, and green chilies (if using) to the pot. Cook until the vegetables soften, about 3-4 minutes.
Pour in the coconut milk and water, stirring to combine. Bring to a gentle simmer.
Add the fish pieces to the pot and season with salt and black pepper to taste.
Simmer gently until the fish is cooked through and the flavors meld together, about 10-15 minutes. Be careful not to overcook the fish.
Stir in the lemon juice and adjust seasoning if necessary.
Remove from heat and garnish with fresh cilantro or parsley if desired.
Enjoy your meal!