Preheat oven to 230°C/446°F.
Prepare the fish. Remove its scales with a fork. Open the belly and remove the fish innards. Rinse well
On your working surface lay a large piece of aluminum foil. On top add a large piece of parchment paper. Put the fish on top.
With a sharp knife make a lengthwise incision at the center of the fish's belly. (on both sides)
Season well with kosher salt. Add some in the belly and at both the incisions you just made.
In a small bowl whisk together the olive oil and lemon juice. Pour half of it over the fish.
Cut one garlic clove into very thin slices and sprinkle over the fish.
Season using half the amount of oregano and some ground pepper.
Fold the parchment paper at each end. Start by folding envelope-like and than secure tightly. Wrap the aluminum foil well around the fish, making sure there aren't any holes for the steam to escape.
Repeat process for the next fish . Add the fishes in a baking pan.
Add about 2 fingers of water in the pan. Bake for 50 minutes to 1 hour.
NOTE: At half an hour check to see if there is still any water left in the pan. If not add a bit more so the fish won't stick to the bottom.