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Char the poblano chiles over a gas flame, turning with tongs until all sides are charred, about 5 minutes. Place chiles in a plastic or paper bag and close tightly. Let sit for 15 minutes, then remove chiles from bag and wipe off charred skin. Carefully slit open chiles lengthwise and discard seeds and stem. Rinse lightly with water. Stuff chiles with equal portions of jack cheese and queso fresco. Do not overstuff, if necessary, pin opening in chiles with toothpick. Whip egg whites until very fluffy, mix in egg yolk. Heat vegetable oil in large skillet. Dredge chiles in flour, shaking off excess, then in egg mixture. Fry chiles evenly on each side until golden, transfer to paper towel lined platter. Lower heat to medium low. Remove oil from skillet leaving about 2 tablespoons oil and any batter left over from frying the chiles. Add all but 2 tbsp. of the onions to the hot skillet, raise heat to medium and fry onions, stirring occassionally. Meanwhile, place remaining onion, serrano (if using), garlic cloves, oregano and tomatoes in a blender and blend for about 1 minute. when onions begin to brown, add sauce to skillet and cook for about 10 minutes, stirring occassionally. Add salt and pepper to taste to sauce. Arrange chiles on top of sauce in skillet, cover, reduce heat to medium low to allow cheese to melt through, 5 to 10 minutes. When done, place 2 chiles on each plate and serve with sauce poured over them.
