Preheat the oven to 200°C, fan 180°C, gas mark 6. Pour the white wine sauce into a large pan. Bring to the boil, then turn down to a gentle simmer.
Add the fish pie mix and cook for 3 minutes, then carefully stir in the tomatoes and peas. Cook for 1 minute, then pour into a baking dish.
Spoon the mash on top and fluff up with a fork. Cook in the oven for 20 minutes until the potato is golden and crisp, then serve.