Try making your own sardine-flavoured ketchup for this delicate steamed fish and tomato dinner for two.
First make the ketchup. Heat a large pan over a medium heat and add a drizzle of olive oil. When hot, add the onions and garlic and cook for two minutes until the onions start to turn translucent.
Add the tomatoes, sugar, bay leaves and chopped rosemary to the pan and cook for 15 minutes until the tomatoes have broken down. Continue to cook until the tomato liquid has reduced right down in volume, almost to nothing.
Now add the wine vinegar and sardines and boil for five minutes. Remove the bay leaves.
Transfer the contents of the pan to a blender or food processor and blend until smooth, then pass through a sieve into a bowl. Taste for seasoning, adding salt and pepper as required. Transfer the ketchup to a clean container and allow to cool. (This ketchup will keep in the fridge for up to three days or you can freeze it for up to a month.)
To cook the fish, heat up a steamer (or place a steamer basket over a pan of simmering water). Take two pieces of baking parchment and fold and scrunch up the sides slightly. Place them on the steamer.
Halve the tomatoes lengthways and sprinkle over the garlic, rosemary and a drizzle of olive oil, then season with sugar, salt and pepper. Place the tomatoes on the parchment in the steamer and steam for five minutes.
Season the bass fillets, oil them all over and then lay them skin side up, on the parchment in the steamer with the tomatoes. Steam for six more minutes until cooked through.
To serve, lift the fish and tomatoes on the parchment from the steamer. Using a spatula, carefully transfer them to warm plates. Drizzle over all the cooking juices from the parchment and serve, with peppery rocket leaves and the ketchup on the side.
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James Martin presents with help from chefs Jason Atherton and Russell Norman.