Preheat Oven To 400 Degrees F (200 Degrees C). Fill The Cavity Of The Trout With The Garlic, Onion, And Lemon Juice. Rub Trout With Butter, And Place On A Large Piece Of Foil. Season With Garlic Salt, Salt, And Pepper. Place The Jalapeno On Top Of The Trout. Seal Trout In The Foil. Bake Trout 25 Minutes In The Preheated Oven, Or Until Easily Flaked With A Fork.