Heat the fat in a frying pan, fry the goldback fillets at medium heat for 13-15 minutes, turning several times. Cook deep-frozen peas and carrots in little boiling salted water for about 15 minutes at medium heat.
Add 1/2 litre water, bring to the boil. Add milk and some salt, stir in the puree flakes. Stir again after about 1 minute. Mix yoghurt, crème fraîche and mustard. Season to taste with salt and pepper.
Arrange 2 fish fillets each with puree and sauce on a plate. Garnish with parsley and lemon.