Rinse and clean the fish as described in the post.
Mix the olive oil, salt, pepper, oregano, chilli pepper and half a lemon's juice in a bowl.
Spread the mixture all around the fish, including the cavity with an egg brush.
Stuff each fish cavity with a parsley spring and a garlic clove and several slices of onion.
Place the fish in a single layer in an oven-proof tray.
Roast in a preheated oven at 230°Celsius (440° F) for about 20 minutes, depending on the size of the fish.
Measure the internal heat of the fish to reach 63°Celsius (145° F). If not available see that the fish is crispy and bubbly on the outside and flakey and white on the inside.