Start with a cold skillet with a cover.  Again, I prefer cast iron for this one.  Line the entire bottom of the skillet with bacon strips.  place as tightly as possible.  Place your Brown Trout fillets on top of the bacon.  Top the fillets with onions and mushrooms.  Top that with another layer of bacon and cover the pan.  Place on stove over medium heat until fish is done.  The bacon will NOT get brown like this.  You are basically steaming the fish with bacon for extra flavor.  If you like, when the fish is done, continue cooking the bacon with cover removed to brown it.  Great with scrambled eggs for a hardy breakfast or with fresh veggies for dinner.