Heat a dry skillet over medium heat. Add the dried chili de árbol peppers and toast them for 30–60 seconds, turning frequently, until fragrant and slightly darkened. Be careful as they burn quickly. Remove from heat and set aside.
In the same skillet (or over an open flame for extra smokiness), char the tomatoes, garlic (still in skin), and onion until softened and blistered, about 8–10 minutes.
Peel the garlic and place it in a blender or food processor along with the roasted tomatoes, onion, toasted chilies, vinegar (or lime juice), salt, sugar (if using), and about ¼ cup of water. Blend until smooth.
Heat 1 tablespoon of oil in a small saucepan over medium heat. Pour in the blended salsa and simmer for 5–7 minutes, stirring occasionally, until slightly thickened and the flavors meld.
Add more salt, lime, or water to reach your preferred flavor and consistency.