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Heat the oil in a sauté pan over a high heat until very hot, then carefully add the peppers and onion. Cook for 5 minutes, or until they have just started to char and blacken at the edges, then turn the heat down to medium and add the garlic. Cook for 3–5 minutes until softened.
Add the tomatoes, chilli flakes and some salt and pepper and give everything a good stir. Pop the lid on and cook for 10 minutes.
Gently nestle the fish fillets into the sauce. Put the lid back on and cook over a medium heat for 14–18 minutes or until the fish is cooked through.
Sprinkle over the basil, pushing some of it into the sauce if you can, then serve.
The cooking time for fish fillets can vary a lot depending on their thickness: thinner fillets may cook more quickly and break up in the sauce if cooked for longer. Just check and keep an eye on them and remove from the heat when cooked through.
Frozen fish fillets are almost always cheaper than fresh. Use frozen fish and adjust the cooking time according to the pack instructions (or defrost first if recommended on the pack). You could also use a cheaper white fish like basa.
The heat of dried chilli flakes varies a lot. Start with just pinch and if you want to add more as the sauce cooks, you can.