Wash the fish and dab dry. Scratch skin several times. Wash herbs. Place 1 clove of garlic, 1 sprig of rosemary and 1 stalk of basil in each fish's abdominal cavity. Place the giltheads and lemon slices on a baking tray lined with baking paper.
Drizzle 2 tablespoons of oil over it and sprinkle with coarse salt. Roast in the preheated oven (electric cooker: 200 °C/ convection oven: 175 °C/ gas: level 3) for 20-25 minutes. Wash the potatoes thoroughly, dab dry and, depending on size, cut in half or in slices.
Heat 2 tablespoons of oil in a large frying pan. Fry the potatoes and 2 cloves of garlic for about 20 minutes at medium heat until golden brown. Turn from time to time. Clean, wash and chop the spring onions.
Wash and halve the tomatoes. Chop 1 sprig of rosemary. Add the spring onions and tomatoes to the potatoes about 5 minutes before the end of the frying time. Season with salt and pepper. Arrange the giltheads, potatoes and vegetables.
Garnish with the rest of rosemary and basil.