Clean the fish, wash them well and leave them to drain. In the juice of 1/2 lemon, grate the garlic.
Next, add salt, thyme and olive oil. Stir well. Smear the insides of the fish with the prepared sauce using a brush.
Chop the lemon and onions into crescents and put them inside the fish. Arrange them in a tray and pour the remaining sauce over the fish. Cover the tray with aluminum foil. Bake in a 392°F (200 °C) oven for 45 min., then remove the foil and bake another 15 min.