Put the 900g chicken in a saucepan with the ½ white onion, 2 garlic cloves, bay leaf, and 1½ tsp kosher salt, then add enough water to cover by about 2–3cm. Bring just to a gentle simmer over medium heat, lower the heat, and cook without ever boiling hard until the thickest piece reaches 74°C in the centre, about 12–18 minutes for breasts and 18–22 minutes for thighs. Lift the chicken out, let it cool just enough to handle, and reserve at least 375ml of the cooking liquid. Shred into medium strands, not fine threads, so the filling keeps real texture. That gentle poach is what keeps the meat tender, because a hard boil tightens the fibres and the chicken will bake again later.

Toast the 4 guajillo chiles and the ancho chile in a dry skillet over medium-low heat for 10–15 seconds per side, just until fragrant and slightly pliable, then cover with hot water and soak for 10 minutes until softened. Meanwhile, char the 900g Roma tomatoes, the roughly chopped onion, and 2 garlic cloves on a dry skillet, comal, or under a broiler until the tomato skins blister and the flesh softens. Drain the chiles and blend everything with the oregano, cumin, 1¼ tsp salt, and 250ml chicken liquid until very smooth, adding a splash more only to keep the blender moving. Heat 1 tbsp oil in a saucepan, pour in the sauce carefully, and simmer over medium-low heat for 10–15 minutes, stirring occasionally, until lightly thickened and no longer raw-tasting. If too thick, add a little more stock, and if too thin, simmer longer. That balance is what the casserole depends on, since thin sauce makes watery slices and thick sauce leaves dry layers.

Roast the 4 poblanos over an open flame, under a broiler, or in a very hot dry skillet until the skins are blistered and blackened on most sides. Transfer to a bowl, cover, and steam for 10 minutes, then peel off the loose charred skin, remove the stems and seeds, and cut the flesh into strips. Heat 1 tbsp oil in a skillet, add the sliced white onion, and cook over medium heat for 5–6 minutes until fully softened and lightly golden at the edges. Add the poblano strips and ½ tsp salt and cook 2–3 minutes more, just to combine. That full softening of the onion is what stops raw slices from leaking water into the layers during baking, while the poblanos stay tender, smoky, and not mushy.

Toss the shredded chicken with about 250–300ml of the red sauce, enough to coat it well and keep it moist but not soupy. Pour the 60–80ml oil into a skillet in a thin layer and fry the tortillas one at a time for about 10–15 seconds per side, just until flexible and slightly strengthened, then drain on paper towels. They should bend easily and must not turn into crisp chips. Mix the two cheeses together, and if the 120ml crema is very thick, loosen it with 2–3 tbsp milk or chicken liquid until it can be drizzled lightly. That light frying is what stops the tortillas dissolving in the casserole, and moist chicken now is what keeps it moist after baking.

Heat the oven to 190°C / 375°F and lightly grease a 23 × 33cm / 9 × 13 in baking dish. Spread a thin layer of sauce over the base, then arrange 6 tortillas, overlapping slightly, to cover it fully. Spread half the chicken over them, add half the rajas, spoon over a little more sauce, drizzle with half the crema, and scatter over one-third of the cheese. Press down lightly, then repeat with 6 more tortillas and the remaining chicken, rajas, sauce, crema, and another third of the cheese. Finish with the last 6 tortillas, a thin layer of sauce, and the remaining cheese, covering the surface evenly. That restraint with the sauce is what keeps every layer moist without flooding, so the casserole does not collapse when sliced.

Bake uncovered for 25–30 minutes, until the cheese is fully melted, the edges are bubbling, and the top is golden with browned patches. If the top needs more colour, broil for 1–2 minutes at the end, watching closely so it does not burn. Remove from the oven and let the casserole rest for 15–20 minutes before cutting. After resting it should slice cleanly while staying moist inside, with visible layers of tortilla, chicken, pepper, sauce, and cheese. That rest is what lets the cheese and sauce set, whereas cutting straight away makes the layers slide apart and the filling run loose.

Cut the casserole into 6 large square portions, marking the lines with a knife first so each slice holds the same number of layers. Lift each portion out carefully with a flat spatula, keeping the layers stacked as they came out of the dish. Drizzle the 2–3 tbsp crema lightly over the top of each portion and finish with a little of the 2 tbsp chopped cilantro. That small drizzle and fresh herb are what add a cool, bright contrast to the rich, savoury interior.