Simmer the potatoes in a large saucepan of water for 15-20 minutes, until soft, then drain and mash with the half fat crème fraîche. Preheat the oven to 200°C, fan 180°C, gas mark 6.
Meanwhile, cook the broccoli florets in simmering water for 4 minutes. Add the frozen green beans for the last minute of cooking time.
Place the basics fish fillets in a bowl. Drain the broccoli and green beans and add to the bowl. Pour over the cheese sauce and mix with the chopped parsley.
Spoon the fish mixture into a 2-litre dish and top with the mash. Spread over and use a fork to create uneven peaks. Bake for 30 minutes until bubbling and golden on top.
Make it veggie: Try swapping the fish for roasted vegetables from the casserole selection. Chop the 750g peeled vegetables and boil them in a saucepan of boiling water for 10 minutes and drain. Add to the dish at step 3 and continue with the recipe.
Nutrition for veggie option, per portion: 465 cals, 15g fat, of which 6g saturated fat, 8.8g added sugar, 0.5g salt.