After washing and wiping down gutted fish, drizzle melted butter and minced garlic inside trout. Cut lemon in half and squeeze juice while capturing seeds in your hand. Stuff all sliced onions and peppers into the fish and wrap with foil. Set oven to 375 degrees F and cook for 25 minutes until the fish is firm to the touch. If you are grilling, place foil-wrapped fish on a hot grill and cook for 10 minutes on each side.