Heat a pan and toast the peppers on all sides. They may get a few dark spots, but you don’t want them to blacken too much or they may become bitter.
Heat 4 cups of water to boiling. Once boiling, turn off the stove. Place the toasted chilis in the hot water and let sit for 15 minutes, until they are softened.
Remove the chilis from the water (reserving 1 cup of the water the chiles cooked in) and place the chiles in a blender, or food processor. Add the remaining cup of reserved chili water, onion, garlic, Mexican oregano, cumin, pepper, and salt. Blend until smooth. (If using whole peppercorns and whole cumin seeds, you will need to grind them with a mortar and pestle or spice grinder before adding them to the food processor.)
Heat the oil in a deep pan over medium heat and pour the sauce into the pan. Bring sauce to a simmer and cook for 15 minutes, stirring often.
Use the sauce, as desired, or in tamales, birria, soups or chiliquiles.
Storage: Once fully cooled, refrigerate leftover guajillo sauce in an airtight container for up to 5 days. Keep it away from heat and sunlight when on the counter for reheating or serving.Reheat: To reheat it, pop it into the microwave for up to a minute or until warm. You can also heat it in a small pot over medium heat for 6-9 minutes. You can also serve it at room temperature or chilled if you’re using it as an appetizer with chips or a topping for tacos.Freeze: Freeze it in 1/2-1 cup portions so that you don’t have to worry about defrosting a huge batch of salsa later. Thaw overnight in the fridge and then reheat or serve as usual.Reduce the Heat: Add 3 to 4 Roma tomatoes to the recipe in order to reduce the heat. You can also bring down the number of guajillos to 5 to 7 for a milder sauce.Wear Gloves: Using plastic gloves to seed and stem the chilis will help you avoid teary eyes later when you accidentally rub your eyes or scratch your nose.Strain the Sauce: In case you used whole peppercorns or cumin seeds and want to make sure your sauce is super smooth, strain it into the pot before simmering it.