Trim, rinse and cut the leeks into very thin rings. Peel, rinse and finely dice the carrots. Rinse the bell peppers, cut in half, remove seeds and ribs, and cut into thin strips. Trim the broccoli, rinse and divide into florets. Rinse the Nile perch fillets under cold water, pat dry and sprinkle with lemon juice.
Heat some olive oil in a saucepan and saute the leeks in it. Add the carrots and bell peppers, and cook briefly. Pour in the white wine and vegetable broth. Cook the vegetables for about 5 minutes. Add the broccoli and simmer covered for 5-10 minutes until al dente.
Meanwhile, cut the Nile perch fillets into 4 equal pieces, season with salt and pepper, and coat with flour. Heat the remaining olive oil in a frying pan and fry the perch fillets from each side for about 4 minutes.
Season the vegetables with salt, pepper and sugar, stir in the creme fraiche and parsley, and spread on 4 plates. Place the fried perch fillets over the vegetables and serve garnished with parsley.