1Begin by preparing the Dried Guajillo Chiles and Dried Ancho Chiles. Place them in a heatproof bowl, cover with hot water, and let them rehydrate for 20-30 minutes until soft. Drain the chiles, reserving a small amount of the soaking liquid.
2In a blender, combine the rehydrated Dried Guajillo Chiles, Dried Ancho Chiles, Garlic Cloves, White Onion, Achiote Paste, Orange Juice, Lime Juice, Cumin Powder, Dried Mexican Oregano, Salt, Black Pepper, Mayonnaise, and Mustard. Blend until a smooth, thick marinade is formed. If the mixture is too thick, add a small amount of the reserved chile soaking liquid.
3Place the butterflied Red Snapper skin-side down on a large baking sheet or platter. Generously spread the prepared marinade all over the flesh of the Red Snapper, ensuring it is evenly coated. Let the fish marinate at room temperature for at least 30 minutes, or refrigerate for up to 2 hours.
