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.
Notes
Adjust the consistency. You can make this peanut salsa thicker or thinner by adjusting the amount of water to your liking.
Adjust the spice level. This salsa has a medium-hot spice level. To make it mild, use 5 árbol chiles. To make it extra spicy, use 15-20.