Sauté the onion in a large non-stick pan until soft, using a little water to stop the onion catching (about 5 minutes).
Add the garlic, and continue to sauté for another 2 minutes, adding a little more water if necessary.
Add the curry powder, stirring for one minute to so the spices start to release their aromas.
Add the tomatoes with 200 ml / 7 floz of water, bring to the boil and then turn down the heat and simmer gently for 5-10 minutes to thicken the sauce.
Season the fish with salt and pepper and add to the sauce. Cook gently for 5 more minutes or until the fish is just cooked through.
Check seasoning and serve.
Notes
For a budget-friendly option, you could use frozen fish.
Serve with basmati rice or Naan bread (see my recipe for Naan bread here)
You could also sprinkle with fresh coriander and make a simple raita to serve with it, by mixing diced cucumber with plant-based yoghurt and a little seasoning.