Roast poblano chiles over an open flame until blackened
Place charred chiles in a plastic bag to steam for 10 minutes
Peel off the charred skin under running water
Make a slit and remove seeds carefully
Mix Oaxaca and mozzarella cheeses together
Stuff each chile with the cheese mixture
Beat egg whites until stiff peaks form
Fold in egg yolks gently
Heat oil and dip stuffed chiles in egg batter
Fry until golden on both sides
Make a béchamel sauce with butter, flour, and milk
Season sauce with salt, pepper, and nutmeg
Arrange fried chiles in a baking dish
Pour béchamel sauce over chiles
Top with remaining cheese
Bake at 350°F until golden and bubbly