Place the 3 pechugas de pollo sin hueso y sin piel aproximadamente 2 libras total in a Saucepan and cover with 4 tazas caldo de pollo para cocción. Add 1 cucharadita sal and bring to a gentle boil over medium-high heat. Reduce to a simmer, cover, and cook for 15–18 minutes, until the 3 pechugas de pollo sin hueso y sin piel aproximadamente 2 libras total is cooked through and no longer pink in the center (internal temp 165°F). Remove the 3 pechugas de pollo sin hueso y sin piel aproximadamente 2 libras total and let it rest for 5 minutes, then shred it with two forks. Reserve the poaching broth.

Preheat your Oven to 350°F. In a large Skillet over medium heat, melt the 4 cucharada mantequilla sin sal. Add the diced 1 cebolla amarilla picada and 1 pimiento morrón verde picado and cook, stirring occasionally, for 5–6 minutes until softened. Add the minced 3 dientes ajo picados finamente and cook for 1 minute more until fragrant.

Add the 1 lata tomates picados con chiles verdes (Rotel) no escurrir (with their juices), 1 lata sopa de crema de pollo, 1 lata sopa de crema de champiñón, and 1 taza crema ácida to the Skillet. Stir everything together until fully combined and smooth. Add the 2 cucharadita chile en polvo, 1 cucharadita comino molido, 0.5 cucharadita pimentón ahumado, and 1 cucharadita sal. Stir well and taste — adjust seasoning as needed. If the sauce seems very thick, stir in a splash 2–3 tbsp of the reserved chicken broth to loosen it. Remove from heat.

Grease a 9x13-inch Baking Dish with Cooking Spray or a thin layer of 4 cucharada mantequilla sin sal. Spread a thin layer of sauce (about ½ cup) across the bottom of the dish to prevent sticking.

Arrange half of the torn corn tortilla pieces in an even layer over the sauce, overlapping slightly to cover the bottom. Spread half of the shredded 3 pechugas de pollo sin hueso y sin piel aproximadamente 2 libras total over the tortillas, then Ladle half of the cream sauce over the 3 pechugas de pollo sin hueso y sin piel aproximadamente 2 libras total. Sprinkle with half of the 2 tazas queso cheddar desmenuzado dividido and half of the 1 taza queso Monterey Jack desmenuzado dividido.

Repeat the layers: remaining tortillas, remaining 3 pechugas de pollo sin hueso y sin piel aproximadamente 2 libras total, remaining sauce, and finally the remaining 2 tazas queso cheddar desmenuzado dividido and 1 taza queso Monterey Jack desmenuzado dividido on top. The cheese layer on top should be generous and even.

Cover the dish tightly with Aluminum Foil and bake at 350°F for 30 minutes. Then remove the foil and bake for an additional 15 minutes, until the cheese on top is bubbly, golden, and beginning to brown at the edges.

Remove from the Oven and let the casserole rest for 10 minutes before cutting and serving. This helps the layers set so it slices cleanly. Garnish with sliced 3 tallos cebolletas rebanadas finamente and fresh 2 cucharada cilantro fresco picado aproximadamente if desired.