Heat the oil in a skillet over medium heat.
Add the jalapeños, serranos, onion, and garlic and fry for 10 - 12 minutes, or until all ingredients are softened and slightly blistered.
Turn off the heat and let the mixture cool for at least 15 minutes.
Transfer everything to a blender, including the oil.
Add 1/2 cup of water and blend until the salsa is completely smooth.
Pour the salsa into a serving bowl.
Season with salt to taste and thin with the remaining 1/4 cup water if needed to reach your desired consistency.
Serve and enjoy!