This Serrano Pepper Salsa is very common in La Huasteca Region of Veracruz and Tamaulipas, where it is used at breakfast or brunch time for Bocoles (a small corn dough patty mixed with beef lard and stuffed with different kinds of fillings) and over fried eggs and steaks just to mention some.
Using a fork prick some holes into serrano peppers, doing this will help them release the steam and burst while frying them.
Note: If you live in an apartment it is a good idea to turn your kitchen fan now.
Heat the oil in a frying pan over medium high heat and add the serrano peppers, roast them on the preheated pan for about 10 to 15 minutes, blister the pepper until cooked through and slightly blackened (turning occasionally to get an even roasting). Remove the peppers and put them aside to cool for a few minutes.
After the serrano peppers have cooled, place them in the blender with the water and pure just for a few seconds to get a chunky consistency.
Add the onion to the frying pan and cook it until transparent.
Finally add the salsa and let it cook for 4-5 minutes. Add salt to taste and enjoy!