In a small pot, place the onion, garlic and tomatillo with enough water to cover the vegetables. If using a serrano pepper & beef bullion, add it on this step.
Cook on medium heat.
While the tomatillo mixture cooks, on a hot skillet, sauté the guajillo chiles on ½ tablespoon of vegetable oil for about 45 seconds on each side. This is a very fast step as it if it cooks too long, the chile will become bitter.
Remove from skillet and add to the tomatillo mixture.
Repeat this process for the chiles de arbol.
Cook the tomatillos until they start to turn yellow and before they crack. Remove pot from heat and set to cool off.
Remove the tomatillos from the pot and transfer to a bowl to cool off ** (leave the rest of the ingredients in the hot water).
Once the tomatillos have cooled off, add all ingredients to a food processor or blender and blend well. NOTE: Do not add the cooked water yet. Only add a little bit at the time if your salsa turns out too dense.
Taste for salt & pepper and add some according to taste.
Store in refrigerator in an airtight container for up to two weeks.