On the top level of the oven, position a wire rack. Preheat your boiler in the oven to high.
Line the sliced tomatoes and onions on a cookie sheet so that their cut sides are facing down. Roast them for 10 minutes so that they are a bit charred from the edges.
In a pan and heat 1 tbsp of vegetable oil. Then add garlic in it and saute it for 2 minutes. Keep flipping so that they do not burn but toast.
Take a blender, and add the guajillo chillies, brown sugar, and toasted garlic cloves in it. Then add apple cider vinegar and water. Keep it unbothered for 5 minutes to let the ingredients soften.
Then at high speed, puree them for 15-20 seconds. Then lower the speed and add vegetable oil. Give a gentle mix.
Add in the roasted onions and tomato slices, season with kosher salt, and blend again at a high speed until a smooth texture is attained.