Bring chicken stock to a boil. Meanwhile, toast the chiles.
Remove seeds and stems from chiles. Add chiles to stock, remove from heat, and let soak until soft.
While chiles are soaking, broil tomatoes, onion, and garlic until cooked and charred, rotating as needed to prevent excessive burning.
Combine everything in a blender; process until smooth.
Transfer sauce to a saucepan (optionally straining out the remaining solids) and bring to a simmer. Cook until reduced to 2 cups.
Made with ancho chiles as instructed this time, and still dried Greek/Italian oregano. The ancho chiles give this sauce a much more complex flavor, smoky flavor.
Used 3 guajillo chiles, 2 roma tomatoes, and 1/4 tsp. Greek/Italian oregano. Needed a bit more salt. Realized afterwards how much larger ancho chiles are compared to guajillo, so probably should have done twice as many chiles as I did.
Made a 4x batch.