1Prepare the masa slurry: In a bowl, combine the Masa harina de maíz with 300 grams of the Water, mixing until smooth and free of lumps. Set aside.
2Roast the vegetables: Place the Tomatillos, Chile poblano, Chile serrano, White onion, and Garlic on a comal or baking sheet. Roast over medium heat or under a broiler until softened and slightly charred on all sides. For the Chile poblano, once roasted, place it in a plastic bag for 10 minutes to steam, then peel, deseed, and devein it.
3Blend the sauce: Transfer the roasted Tomatillos, peeled Chile poblano, roasted Chile serrano, roasted White onion, and roasted Garlic to a blender. Add 200 grams of the Water and blend until a smooth sauce is formed.