How to Make It
In a medium cast-iron skillet, toast the chiles over moderately high heat until fragrant, about 1 minute. Fill a small bowl with very hot water and submerge the toasted chiles in it until softened, about 5 minutes. Drain the chiles; discard the soaking liquid.
Meanwhile, in the same skillet, cook the tomatoes and onion cut side down over high heat until they begin to blacken, about 3 minutes. Transfer the tomatoes and onion to a blender and add the softened guajillos, garlic and stock; puree until smooth.
In a medium saucepan, heat 1 1/2 tablespoons of the vegetable oil. Add the flour and whisk over moderate heat until smooth. Slowly whisk in the chile sauce and season with salt and pepper. Simmer, stirring occasionally, until the sauce has thickened slightly, about 8 minutes.
Season the cod with salt and pepper. In a large nonstick skillet, heat the remaining 1 tablespoon of vegetable oil. Add the cod and cook over moderately high heat until white throughout, about 4 minutes per side. Transfer the cod to plates, spoon the guajillo chile sauce on top and serve immediately.