Gutting trout and cutting crossways a few times, place in a flat, greased casserole dish. Place some garlic in the incisions, mix the rest with the spring onions and sprinkle over the fish. Cut the ginger into fine strips and place on the fish. Drizzle with soy sauce. Arrange the onion rings and tomato quarters around the fish. Season everything with salt and pepper and drizzle with water. Cover tightly with aluminum foil and cook for one hour at 175 ° C. Take off the foil and water the fish. Cover and cook for another 30 minutes at 135 ° C.