How to cook
Clean the fish of their insides and heads and salt them. Leave them for about 20 min. to take in the salt.
Peel the onions and chop them finely. Put them in a suitable oven dish and add the olive oil. Put the oven dish in a preheated oven. Cook the onions until softened.
If needed, season with salt. Bake the dish until it's left in just oil and thickens.
You can turn the mackerels over carefully at the halfway mark during baking, to crisp the other side as well.
Before they're ready, thicken the sauce with the flour, which you've dissolved in a little water ahead of time.
Serve the dish a piece of lemon.