Heat butter in a large skillet over medium-high heat. Add HALF the onion and the mushrooms and stir briefly to coat. Cover the pan and cook 1-2 minutes or until mushrooms are browned and have released some liquid. Uncover and cook until mushrooms are dry. Stir in spinach, salt and cook until spinach is wilted. Set aside.
Make the suiza sauce:
Peel and rinse the tomatillos. Place in a pot with remaining half of diced onion, cover with water and put over high heat.
Meanwhile, roast or broil the pepper on all sides until blackened. Let cool, then remove the stem (and seeds for mild sauce).
Once tomatillos have turned a mossy green, transfer all to a blender with the onion, garlic, poblano, cilantro, salt, pepper, nutmeg. Carefully blend until smooth. Mix in crema.
Soften tortillas in oil and roll 2-3 tablespoons of spinach mixture into each. Pack into a 9×13 pan.
Pour sauce over all and sprinkle with cheese.
Place under broiler for a minute until cheese melts. (The sauce and filling are already warm so you just need to melt the cheese on top.)