Roast the poblanos over and open flame or 4 inches below the broiler, turning regularly until blistered and blackened all over. About 5 minutes on open flame or 10 minutes in broiler. Place in a bowl, cover with a towel and let cool until handleable.
Place the spinach in a microwavable bowl, cover it with plastic wrap, poke a few holes in the top and microwave on high until wilted, about 2 minutes. Set aside.
Turn the oven on to its lowest setting. Heat the oil in a large skillet (preferable nonstick) over medium heat.
Add garlic and cook til soft and lightly brown.
Put garlic in blender.
Rub the skin off the chiles and pull out the stem and seed pods. Rough chop and add to blender with the masa harina and milk. Blend till smooth.
Return skillet to medium high meat. Sprinkle both sides of the fish with salt and pepper. Lay the fillets in the hot oil and cook until richly browned, about 2-3 minutes.
Use a spatula to flip the fillets and cook until barely flakes when pressed firmly with a finger or the back of a spoon. You want it underdone.
Move fish to oven safe plat and set in oven.
With skillet still over medium high pour the mixture from the blender and whisk until it comes to a boil and thickens -- about 1 minute. Reduce heat to medium low and simmer, stirring frequently for 5 minutes. If thicker than cream, add a bit more milk. Taste and season with salt and pepper.
Add spinach to the sauce and stir until it is warm and well coated.
Plate fish with sauce under or over, your preference.