Toast guajillo chiles for about 20 seconds per side over an already hot pan or comal set over medium low heat. Be careful not to burn them or they will taste bitter.
Place toasted chiles, tomatoes and garlic in a pan covered with hot water and simmer for about 10 minutes until the guajillos are softened and tomatoes are cooked through. Place the chiles, tomatoes and garlic in the blender with about ½ cup of the cooking liquid, the onion, oregano, cumin and salt and puree until smooth. Strain the sauce.
Heat oil in a sauce pan set over medium high heat. Once hot, pour in the sauce, careful because it will jump a bit, and simmer for about 12 to 15 minutes, or until the sauce thickens and seasons. Taste for seasoning and add more salt if needed. Once cooled down, the sauce may be refrigerated for a couple weeks.