Pour wine and vinegar into a pan and bring to the boil, add fish fillets, cover and simmer gently until just cooked.
Cut avocado in half, remove seed and skin, cut flesh into dice and place into a bowl.
Add tomato, onion, parsley and season to taste.
Lift fish out of the pan onto plates, spoon salsa on top.
Bring cooking liquor to the boil and reduce by half, whisk in the butter.
Sieve sauce through a fine strainer and pour over the top of fish and salsa.
Serve with boiled new potatoes and a fresh salad or some baby vegetables.
Serving Size 4