Heat the vegetable oil in a large pan over medium heat.
Add the chopped onion and sauté until golden brown.
Stir in the minced garlic and grated ginger, cooking for 1-2 minutes.
Add the pureed tomatoes and cook until the mixture thickens.
Sprinkle in the turmeric, cumin, and coriander powders, stirring well.
Pour in the coconut milk and stir until combined.
Bring the mixture to a gentle simmer.
Add the fish fillets, ensuring they are submerged in the sauce.
Cover and cook for 10-15 minutes, or until the fish is cooked through.
Finish with garam masala and salt, stirring gently to combine.