Melt half the butter in a skillet. Cook the onion in it for 5 minutes. Add mushrooms and cook 5 minutes longer. Mix parsley, crackers, 1 teaspoon salt, 1/4 teaspoon pepper and thyme. Split the trout for stuffing and stuff with prepared mixture. Sew the openings. Mix the flour with remaining salt and pepper. Roll trout in it. Melt remaining butter in skillet. Cook the trout in it over low heat, turning until browned on all sides. Bake 20 minutes in 350 degree oven. To serve, sprinkle with lemon juice. Serves 4.