Chop the onion and crush the garlic
Heat up a pan with olive oil and cook the onions until they are transparent. Add the garlic and stir for a few seconds
Add the cod. When it start cooking you can press with a wooden spoon and separate the flakes.
Add the flour and stir until the flour has some color. Start adding the milk slowly while you stir. The goal is to reach a consistency such as a thick porridge. Add salt if the cod was not salted.
Let it cool down, and then fill the piquillo peppers