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.
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.