Adjust oven rack to middle position and heat oven to 300°F. Arrange chiles on rimmed baking sheet and toast until fragrant and puffed, about 8 minutes. Transfer to plate and let cool, about 5 minutes. Stem and seed chiles, and break into small pieces. Working in batches, grind toasted chiles, MSG, and oregano in spice grinder or with mortar and pestle until finely ground. Add cumin and stir to combine.