Put the fish fillets on a plate and let them thaw. Bring 1/4 litre of water to the boil, stir in sauce powder and boil for 1 minute. Drain the mandarin oranges, catching the juice.
Stir 2 tablespoons of juice into the sauce. Clean and wash the spring onions and cut them into fine rings. Clean, wash and slice the mushrooms. Add mandarin oranges, spring onions and mushrooms to the sauce.
Sprinkle thawed fish with lemon juice. Place in a small casserole dish, pour sauce over it. Sprinkle with crushed pink pepper. Bake in a preheated oven (electric: 200 °C/ gas: level 3) for 20-25 minutes.
Sprinkle with peppermint leaves if necessary.