Cook the tomatillos along with the serrano peppers.

Once cooked, let them cool (be careful not to overcook as they can become bitter).

Finely chop the white onion into small cubes and chop the cilantro.

Blend the tomatillos and peppers well, and don't forget to add salt.

In a bowl, combine the sauce with the chopped onion and cilantro.

Dice the avocado, add it, and enjoy.

This sauce is that simple. It can also be made without the peppers and still tastes great.