I make fish soup with frozen fish fillets or fish pie mix. I cook onion, fennel or carrot and celery in oil until soft. Then, depending on which kind I'm cooking, I either:
(i) add a little veg or fish stock plus dicde potato if wanted, simmer until the potato is cooked. Then add the fish, turn heat down and cook for a further few minutes until the fish is cooked, finishing with a dollop of creme fraiche; or
(ii) add passata and diced potato or courgette, simmer until cooked. Then add the fish, turn heat down and cook for a further few minutes until the fish is cooked.
So either creamy fish soup or tomato-y fish soup.
In both cases, I flavour it with black pepper and fennel seed and saffron if I've got any. Tarragon is nice in both, too.
Thai-style fishcakes I make by blitzing chopped salmon (can use frozen for this too, and you don't need to thaw it before blending) with lemongrass paste, chopped spring onion, grated ginger, white pepper, a splash of fish sauce and an egg. Fry.
Kedgeree is always great, and you don't really need to use loads of fish.
Don't forget smoked mackerel pate: blend skinned smoked mackerel fillets (preferably peppered ones) with lemon juice, powdered mace, paprika, cream cheese and a dollop of mayonnaise.
All these need herbs and spices which aren't cheap, but I find they do really help jazz up cheap ingredients.