Broil the Vegetables - Cut 3 Roma tomatoes and 1 onion into large chunks. Peel 16 garlic cloves and place everything on a baking tray. Broil for about 10 minutes, turning halfway, until soft with charred edges. Watch the garlic to prevent burning.
Prepare the Dried Chiles - Remove the stems and seeds from 2 guajillo chiles and 1/2 cup chile de árbol. Wear gloves when handling the chiles and avoid touching your face. Use fewer chile de árbol peppers for a milder sauce.
Toast the Chiles - Heat a dry skillet over medium-low heat. Toast the prepared chiles, turning often, until fragrant. Avoid burning them, as this can make the sauce bitter.
Soak the Chiles - Place the toasted chiles in a blender or heat-safe bowl. Add 1/2 cup water and 1/3 cup apple cider vinegar. Let them soak for 10 minutes, keeping all the liquid for blending.
Blend the Chiles and Add the Oil - Blend the chiles and soaking liquid on high for about 1 minute. Reduce the speed to low and slowly pour in 1 cup vegetable oil until well combined.
Add the Roasted Vegetables - Add the roasted tomatoes, onion, and garlic to the blender. Blend until smooth and orange-red, scraping down the sides as needed.
Season and Chill Before Serving - Add salt to taste and blend briefly. Transfer the sauce to a clean, airtight container and refrigerate overnight to let the flavors blend. Keep chilled until serving.
