Heat the oil in a cast-iron skillet. Add the onions and cook over medium heat for 3 minutes or just until they soften a bit.
Add garlic and tomatoes and keep stirring for 4-5 more minutes until everything is nicely cooked.
Make some room in the middle and add the chilies and cumin seeds.
Cook for a couple of seconds just to slightly fry the chili peppers.
Add ¼ cup of water and 1 teaspoon of salt, mix and cover the pan.
Allow to slightly cool down then transfer everything to a blender
Blend at high speed until you’ll have a smooth sauce.
Adjust salt to taste and transfer the salsa to a bowl.
Serve immediately or store it in an airtight container for later.
Notes
To store this salsa:
Transfer it to a glass container with a lid and store it in the fridge for up to 1 week.
Place the sauce in a freeze-proof container or bags and store it in the freezer for up to 2-3 months. When ready to enjoy, thaw it, stir it a bit, and serve at room temperature.