After your fish is totally cleaned, place it in a large baking pan and score it with a knife on each side, making three lines or incisions on its body. You can make it fancy and do another 3 lines the cross way if you're in a Jaques Pépin kinda mood.
Season it generously with salt, pepper, and oregano, especially inside the gutted slit and on the side incisions.
You may also add some garlic or slices in the belly and the incisions.
Place the fish exactly in the middle of the pan. Around it, add the potatoes,onions and tomatoes in a row, alternating one after the other. Season those as well, with salt, pepper, parsley and lots of oregano.
Place any leftovers inside the belly of the fish.
Drizzle the food with the juice of one and a half lemons.
Add the olive oil, evenly coating all ingredients.
Add 1/2 cup water.
Place in a 375 to 400 degree oven and cook until potatoes are tender for about 45 minutes to an hour. You may even foil it first for about half the time.
The sauce shouldn't be too thick or too watery.
The fish and potatoes should be a bit crisp and brown on their surfaces.