First, position a wire rack right on the top level of your oven, about 6 inches from the broiler and preheat the broiler to high.
Place onion slices and tomatoes cut side down onto a cookie sheet, then let them roast until slightly charred around the edges – about 10 minutes should do it!
Next, grab a pan and sauté garlic in 1 tablespoon of vegetable oil for a minute or two, flipping occasionally until they just start to toast (but not burn).
Transfer the chiles and garlic over to a blender, pour in some water and vinegar, and let it sit for 5 minutes so they soften. Now, purée on high for 10-20 seconds before switching to low speed and pouring the remaining vegetable oil in a slow and steady stream. This will help create the creamy consistency you’re looking for!
Throw in the tomatoes, onion, and salt before blending it up one last time on high until smooth.And voilà – your delicious chile sauce is ready to serve!