Preheat the oven to 200C/400F/Gas 6.
Heat a deep frying pan over a medium heat.
Add the oil, onions and garlic and cook for eight minutes, until the onion is soft.
Add the tomatoes, stock and seasoning, and stir to combine.
Cook uncovered until the mixture has reduced and thickened slightly, which should be about ten minutes.
Place the fish in a baking dish, top with the tomato mixture and cook in the preheated oven for 12 minutes.
To serve, remove the fish from the sauce, and keep warm.
Stir the spinach and basil into the sauce, and cook until the spinach has wilted.
Spoon the sauce over the fish and serve immediately with the boiled new potatoes.