1MAKE SURE THE ROOM IS WELL VENTILATED BEFORE STARTING THIS STEP! Toast the chilies in a dry large cast-iron skillet over med-hi heat, turning until warm and soft, about 30 seconds per side (do not blacken, or they will be bitter.)
2Discard stems; cut chilies lengthwise with shears, and discard seeds. Cover chilies with the boiling water. Let stand until hydrated, about 20 minutes.
3Meanwhile, cook garlic and tomatoes in skillet over med-hi heat, turning, until charred and soft, about 10 minutes. Set tomatoes aside; peel garlic.
4Heat oil in skillet over med-low heat until hot but not smoking. Cook onion until translucent, about 5 minutes. Add chili powder; cook 1 minute.
5Drain chilies, reserving 1 1/2 cups liquid.
6Puree onion, hydrated chilies, chipotle chilies, tomatoes, garlic and oregano in a blender, adding a small amount of the reserved soaking liquid, if the mixture sees dry. Pass sauce through a fine sieve into skillet.
7cook over medium heat, stirring 5 minutes. Add remaining soaking liquid. Simmer until sauce is thickened, 12-15 minutes. Season with salt. If not serving immediately, refrigerate up to 2 days. Just before serving, heat over low heat; add water if sauce seems too thick.