Add tomate de arbol and serrano chiles to bowl of blender. Puree until smooth and combined, about 30 seconds. Transfer chile mixture to small saucepan over medium-high heat. Add ¼ cup water; bring to boil. Cook, stirring occasionally, until mixture reduces to 1 ½ cups, about 5 minutes.
Remove chile mixture from heat. Let sit until just warm to the touch. Stir in onions and cilantro. Season with Adobo, to taste.
Ecuadorian Hot sauce can range in heat from slightly hot to extra spicy. To make a sauce that’s mild, remove and discard the white inner membrane and seeds; this is where much of the spice in the chile pepper is stored. For medium heat, add only a portion of the seeds, and for a super fiery ají, do like the brave, and add it all.