Combine flour, salt, olive oil, and beer in a bowl and mix well.

Cover the batter and let it sit at room temperature for 30 minutes to allow slight fermentation.

Whisk egg whites until stiff peaks form.

Gently fold the beaten egg whites into the batter.

Remove any skin or bones from the cooked fish.

Mash the boiled potatoes.

Mix the fish, potatoes, chopped garlic, herbs, and softened butter together in a bowl.

Add the egg yolks one at a time, ensuring the mixture holds its shape without being too dry or liquid.

Season the mixture to taste.

Take a tablespoon of the fish mixture and roll it between your hands to form a small ball. Flour your hands as needed to prevent sticking.

Repeat the process until all the mixture is used.

Dip each fish ball into the prepared batter using a fork.

Carefully plunge the battered fish balls into hot fat.

Fry until golden brown and crispy.

Remove the fried fish balls and serve with tomato sauce or mayonnaise.