Pat the fish fillet dry and sprinkle all over with lemon juice. Rub generously all over with the soft garlic butter. Season to taste with the salt and pepper and dust lightly all over with a little flour then generously rub all over with mayonnaise.
Place a layer of lemon slices in a baking tray and position the fish fillet on top of the lemon slices, the skin side down. Spread a little more mayo on top of the fish and bake in the oven preheated to 190 ºC for 5 – 7 minutes or until the fish is just cooked. Remove from the oven and serve.
Fish should be baked at 190°C (preheated) for 5 minutes for every centimetre of its thickest part – no longer.