Remove seeds and dice tomatoes.
Peel potatoes and boil in salted water until tender and strain them. Using an electric hand whisk, start whisking at a slow speed to break up the potatoes, then increase the speed to fast and whip them up to a smooth purée. Stir in hot milk. Season with salt, pepper and pesto. Fold in diced tomatoes.
Fry fish fillets crispy and serve with mashed potatoes.