Toast the stemmed, seeded chiles in a dry skillet over medium heat, pressing them flat with a spatula for 5 to 10 seconds a side, just until fragrant and slightly softened. Remove them before they darken, as scorched chiles turn the sauce bitter.

Put the chiles in a bowl, pour 2 cups of the hot broth over them, weigh them down with a small plate, and soak for 15 minutes until fully soft.

Blend the chiles, their soaking broth and the garlic until completely smooth, at least a full minute. For a very silky sauce, press it through a sieve.

Heat the oil in a saucepan over medium heat, whisk in the flour, and cook, whisking, for about 1 minute until it smells toasty and turns pale gold.

Add the cumin and oregano and whisk 15 seconds, then pour in the chile puree carefully, as it will spit.

Whisk in the remaining cup of broth, the vinegar and the salt. Simmer gently for 10 to 15 minutes, stirring now and then, until it coats the back of a spoon.

Taste and adjust the salt. Use warm, or cool and refrigerate.