Have your fishmonger prepare 12 trout fillets (2 per person).
Clean the mushrooms: sort, rinse quickly under running water, and gently squeeze dry.
In a non-stick pan, melt 30 g butter and sauté mushrooms over high heat for 5 minutes until water releases. Drain, return to pan with 20 g more butter; season with salt and pepper.
Preheat oven to 180°C (gas mark 6).
Cut 12 aluminum foil rectangles. Place a seasoned trout fillet on each, top with mushrooms and a butter knob. Seal packets. Bake 15 minutes.
Wash lemons; slice into 12 pieces.
Serve in foil packets to preserve heat and aromas, with a lemon slice atop each fillet.
