Rinse the fish filets under a thin stream of cold water, drain and pat dry. Place in a container and drizzle with the lime juice, garlic, S & P. Let the fish marinate 15 minutes up to 2 hours in the fridge.
Slice the poblanos in half, removing the stem, seeds, and veins. Roughly chop and place in blender along with the milk and blend until smooth.
In a saucepan over medium heat, melt the butter and add the flour to make a roux. It should be nice and foamy. Cook until golden, 2-3 minutes. Add the chile puree, Mexican crema and nutmeg and cook on low heat until it thickens, about 10 - 12 minutes. Season with S & P.
Preheat the oven to 375 degree F. Butter the bottom of a baking dish and place the marinated fish in the dish - discard marinade. Cover generously with the poblano sauce. If using cheese, sprinkle it on top.
Bake just until the fish is cooked through and it flakes with a fork, 15-20 minutes, depending upon the thickness of the filets.