Preheat your oven till it gets hot.
Place a cookie sheet on your oven pan, and keep the sliced onion and tomatoes with cut side down. Let it roast for about 10-15 minutes until they get a bit charred on the edges.
Get a pan and place it on a cooker on medium heat, fry the garlic quickly in a tablespoon of vegetable oil for about a minute. Add chiles to garlic, fry them quickly for 1-2 minutes while flipping them frequently until they just start to toast, avoid burning
Take the fried chiles and garlic to a high-speed blender, and add the water and vinegar, let them sit for about 5 minutes so they can become soft.
Blend them on high speed for 15-20 seconds, switch to low speed, then add in the remaining vegetable oil slowly, and let the mixture reach a creamy consistency.
Now, add the roasted tomatoes and onion with salt to the mixture and blend them all together on high speed until it is smooth.
The sauce is now ready, serve it with tortilla chips or add it to tacos. You can store it in a jar, and place it in the fridge for about 2 weeks. Enjoy!