Heat a large pan with olive oil and sauté onions and garlic until translucent.
Add 1kg tomatoes, 75g sugar, bay leaves, and rosemary, and cook for 15 minutes.
Add vinegar and sardines, boil for five minutes, and remove bay leaves.
Blend the mixture until smooth, sieve, and allow to cool.
Prepare a steamer with two pieces of scrunched baking parchment.
Halve plum tomatoes, season with garlic, rosemary, oil, sugar, and salt, then steam for five minutes.
Oil and season the bass fillets, place them skin side up in the steamer, and steam for six minutes.
Transfer fish and tomatoes to plates and serve with rocket and sardine ketchup.