Clean the fish. Make two notches lengthwise and add salt, pepper, oregano and 1-2 slices of lemon.
Season the belly of the fish with salt and pepper and add a slice of lemon.
Line a baking tray or casserole dish with parchment paper and place the fish in it. Pour a little oil on top. Bake at 200-220 degrees on the grill or air resistance for 20 to 25 minutes, turning once.
Transfer the fish to a platter, and add a little more oregano.
Whisk in the oil lemon, pour over the food, and sprinkle with chopped onion and parsley.
Allow a little time for the fish to draw oil and serve.