Add the jalapeños, tomatoes, onion, garlic, salt, and black pepper into a blender and blend until smooth.
Heat the oil in a small saucepan over medium-high heat. Carefully stir in the blended salsa and cook for 5 minutes, stirring occasionally.
Transfer to a serving bowl and let it cool down slightly, or cover and chill in the refrigerator for 10 minutes.
Stir in the diced avocado right before serving.
Video
Notes
This salsa is a medium or hot spice level. If you prefer a mild spice level, remove and discard the seeds and veins of the jalapeños.
Seasoning is crucial in this salsa. The ingredients are fresh and relatively basic; be sure to taste as you go, and ensure you have the proper salt level, as it makes a big difference.