Preheat oven (electric cooker: 200°C/circulating air: 175°C/gas: level 3). Clean or peel the vegetables, wash them, cut them in half lengthwise and then cut them into slices. Mix with salt. Rinse fish and pat dry.
Sprinkle with lemon juice and season with salt and pepper.
Put 1/4 of the vegetables and 1 piece of fish in the middle of each piece of baking paper. Spread butter in flakes on top. Close the packet tightly. Bake in a hot oven for 25-30 minutes.
In the meantime, clean and wash the salad, spin dry and cut into strips. Peel the mango and dice it roughly. Finely puree mango, vinegar and oil. Season to taste with salt and pepper. Mix salad and mango sauce.
Pass the salad with the fish.