In skillet, melt 3 tbsp. butter, add shallot and saute until tender. Add mushrooms and cool a few minutes until moisture is evaporated. Add tomatoes and cook 5 minutes longer. If mixture is very watery, turn up heat and cook until liquid evaporates and tomato mixture is fairly thick. Season with salt and pepper, remove from heat.
Pat trout dry inside and out with paper towels. Season lightly with salt and pepper and dust with flour. Heat 2 tbsp. butter in a large skillet. Cook trout over medium heat about 8 minutes per side or until done; a few minutes before the fish is done, reheat the mushroom mixture on top. Sprinkle with parsley.
Pour off fat remaining in skillet. Add 3 tbsp. butter and lemon juice. Heat until butter is melted. Pour over trout.