In a dry skillet or pan, toast the guajillo chiles over medium heat for 2-3 minutes until fragrant. Remove the stems and seeds.
Boil the tomatoes, garlic, and onion in water for about 5 minutes, until the tomatoes begin to soften.
Remove the tomatoes, garlic, and onion from the pot and transfer them to a blender. Add the toasted chiles, salt, and a little of the water from the boiling process. Blend until smooth.
Pour the salsa into a saucepan and simmer for 5-10 minutes. You can add vegetable oil for a smoother texture, if desired.
Taste and adjust salt as needed. Serve with carnitas tacos.
