Preheat the oven to 220°C, fan 200°C, gas mark 7. Grease a non-stick baking tray with ½ tablespoon of olive oil.
Bring a large pan of water to the boil and boil the potatoes for 10 minutes, then drain. Leave to steam dry in the colander for a further 10 minutes.
Put the fish, spring onions, egg, mustard and 2 tablespoons plain flour in a large bowl. Grate the cooked potato on top, using the coarse side of a grater.
Mix well and shape into 8 fishcakes. Place the remaining plain flour on a plate and coat each fish cake, shaking off any excess.
Arrange the fish cakes on the oiled baking sheet, and brush the tops and sides with the remaining olive oil. Bake for 25 minutes.
Meanwhile, bring a large pan of water to the boil and cook the carrots and peas until tender, then drain. Flip the fish cakes over to serve bottom side up with the vegetables.
Make it veggie: Try swapping the haddock for 100g Sainbury's Sundried tomatoes in vegetable oil, chopped.
Nutrition for veggie option, per portion: 603 cals, 24g fat, of which 3.4g saturated fat, 18g added sugar, 0.9g salt.