This is probably the most versatile of Mexico's many red chile sauces. It also shows two important techniques. To develop the flavor of the dried chiles while softening them and taking away any bitterness, they are washed, quickly heated on a griddle, and soaked in hot water. Learn to recognize the moment when they are fragrant but not scorched. Scorched chiles will turn the entire dish bitter and unusable. The other technique to note is one that I have made an even more special point of in my own cooking: After the sauce has been puréed, it is finished by being cooked in a small amount of hot fat. This kind of final sautéing is an important addition to flavor. It both deepens and melds the effects of separate ingredients. This particular chile sauce is also bound with a roux, which is not true of all.
Besides being used for picadas, this sauce can be used to make enchiladas rojas (red enchiladas), with any filling you like. It is used with cooked pork to make carne con chile colorado and to season pozole, menudo, or Northern-style tamales. We also used it at my restaurant with string beans or potatoes to make simple Lenten dishes. In that case the sauce would have been cooked with vegetable oil and water instead of lard and chicken stock.
Heat a heavy skillet or griddle over medium-high heat. Meanwhile, remove stems and seeds from chiles while rinsing under cold running water. Place them on griddle and toast, 3 or 4 at a time, just until the aroma is released, 30 to 60 seconds. Be careful not to burn them. Place the chiles in a bowl and cover with hot water. Let soak until softened, about l0 minutes. Drain chiles and discard liquid.
Place chiles, oregano, 2 garlic cloves, and 2 cups of water or stock in container of blender and process to a smooth purée. Add more stock if it is too thick for the blender. With a wooden spoon or pusher, work the purée through a sieve into a bowl, pushing and scraping to get all the solids. You may want to pour in a little more liquid to help rinse the sauce through the sieve.
In heavy medium saucepan, heat lard over medium-high heat until rippling. Add remaining garlic clove and brown in the hot fat, pressing down with the back of a cooking spoon to release the flavor. Remove and discard garlic.
Add flour to hot fat and cook, stirring constantly, until golden. Add the strained chile purée to the pan and reduce the heat to low. It will splatter as you pour it in — be careful. Cook over low heat, stirring often, until raw taste is gone and flavor of chile is mellowed, about l0 minutes.
Can be stored tightly covered in refrigerator for up to a week or indefinitely in freezer.