Start by cleaning the salmon, removing the fins.
Make slits all over it, about 3-finger distant from each other and also cut it in half lengthwise.
Flip the salmon and repeat the same slits on the other side.
In a bowl, add the oil, the chili flakes, the sweet paprika and the salt, mix well. Add the lemon juice and mix again.
Using a brush, spread this seasoning mixture on both sides of the fish and inside of it.
Cover the fish with a plastic bag and let it marinate for 30 minutes.
Meanwhile, cut the onions and the lemons into round slices. Cut the tomatoes into small equal pieces.
Once the fish has marinated for the indicated time, on a large baking sheet make a layer with the onion slices. Add the lemon slices and cover them with the seasoned fish.
Put 1 bunch of fresh parsley inside the fish.
Top the fish with the butter chopped into strips.
Place the tomato wedges around the salmon and cover it all with foil.
Take to a preheated oven at 200°C (392°F) and bake for 1 hour.
Serve.