Prepare the fish by opening its belly and removing the innards. Rinse well.
Preheat oven to 200°C / 392°F.
Lay the potatoes in a 28 x 18 cm (11 x 7-inch) baking dish. Add the onion and half the garlic on top. Season with salt, pepper, and ½ tablespoon dried oregano. Drizzle the lemon juice on top.
Stuff the belly of each fish with 2 slices of lemon and a few slices of garlic.
Place the fish on top of the potatoes. Drizzle the olive oil and remaining oregano on top.
Add ¼ cup water to the dish from the side (don't rinse the potatoes or the fish).
Cover the baking dish with aluminum foil and bake for about 50 minutes.
Remove from the oven. Turn the oven to broil function (see note 1 below). Uncover the dish. Place the baking dish on the top rack of the oven.
Broil the fish for 7-10 minutes or until it looks nicely roasted on top.
Let stand for 10-15 minutes to rest (you may cover it again with the foil) then serve with an extra squeeze of fresh lemon juice and dried Greek oregano if you like.
Notes
Note 1: If your oven doesn't have a broil function, grill function, or air will do as well. If using grill or air you may need to add an extra splash of water to avoid drying. Also, use a high temperature and roast quickly so the fish won't dry out.