Rinse the fish filets under a thin stream of cold water, drain and pat dry. Place in a container, drizzle with the lime juice, garlic, salt and black pepper. Let it marinate anywhere from 15 minutes up to two hours in the refrigerator.
Slice the poblanos in half, removing the stem, seeds and veins. Roughly chop and place in the blender along with the milk, purée 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 to 3 minutes. Add the chile purée, Mexican cream and nutmeg and cook on low heat until it thickens, about 10 to 12 minutes. Season with salt to taste.
Preheat the oven to 375 degrees Fahrenheit. Butter the bottom of baking dish and place the marinated fish there, without extra marinade. Cover generously with the poblano sauce. If using cheese, sprinkle it on top.
Bake just until the fish is cooked and flakes with a fork, 15 to 20 minutes, depending on the thickness of the filets.