Preheat your oven to 400°F (200°C). Place tomatillos, poblano peppers, quartered onion, garlic cloves, and serrano pepper on a baking sheet. Roast for about 20-25 minutes until tender and slightly charred.
Let the vegetables cool for a few minutes. Transfer them to a blender or food processor. Add ground cumin, salt, baby spinach, and cilantro. Blend until smooth, adding a splash of water if needed.
In a large mixing bowl, combine cooked shredded chicken with half of the blended tomatillo sauce and half of the shredded cheese. Mix well.
Warm the tortillas briefly in a skillet to make them pliable. Spoon a generous amount of the chicken mixture onto each tortilla, roll them up tightly, and place seam-side down in the baking dish.
Pour the remaining tomatillo sauce over the enchiladas, followed by the sour cream and half & half mixture. Sprinkle the remaining shredded cheese on top.
Cover the baking dish with aluminum foil and bake for about 20 minutes. Remove the foil and bake for an additional 10-15 minutes until the cheese is melted and bubbly.
