Heat the olive oil in a large skillet over medium-high heat. Lightly fry the peanuts, garlic and onion for 2-3 minutes, stirring occasionally, until lightly toasted and fragrant. Transfer them to a blender or food processor with a slotted spoon and set aside.
In the same skillet, toast the árbol and guajillo chiles for 30-45 seconds until toasted and fragrant. Try not to burn them or they will get a bitter taste. Transfer them to the blender or food processor.
Add the water and kosher salt to the blender or food processor and blend for 2-3 minutes until completely smooth.
Taste and adjust salt as needed. Serve immediately with tortilla chips or on tacos and tostadas.
