Preheat the oven to 190°C, fan 170°C, gas mark 5. Heat the oil in a large saucepan and fry the onions for 5 minutes until soft.
Add the fish pie sauce, fish pie mix and peas. Stir to combine, then transfer the fish mix to a 3-litre ovenproof dish.
Stir the parsley into the mashed potato and spoon over the top of the fish mixture. Bake for 20 minutes, until the mashed topping is just starting to brown.
Meanwhile, bring a large pan of water to the boil, add the beans and cook for 5 minutes. Serve the hot pie with the drained green beans.