Sauce
Start by removing all the fins from the fish and cut off the tail.
Make some slits on both sides of the fish.
Season it with the salt, the black pepper and the sweet paprika, spreading the spices all over the fish and inside of it.
Cover the fish with plastic wrap and let it rest in the fridge for 30 minutes.
After this indicated time, cover the bottom of a baking sheet with the onions chopped into strips.
Add the fish and stick 1 thin slice of butter into each slit we made. Add some butter slices inside the fish as well.
Cover the fish with parchment paper and aluminum foil.
Bake in a preheated oven at 200°C (392°F) for 40 minutes.
Meanwhile, into a skillet over medium-low heat, add the olive oil and the crushed bell peppers, stir-fry well.
Add the grated tomatoes, the salt, the sweet paprika and the black pepper, stir and let it all cook for 5 minutes.
After this indicated time, remove from the heat.
Once the fish has baked through, add the sauce around the fish, top with parsley to taste, sprinkle the mozzarella cheese over the sauce and take to the oven for 15 minutes.
Serve.