Prepare the chiles: Slit chiles open on one side and soak for 1 hour in water with vinegar and salt to remove some piquancy.
Remove chiles from water and drain and dry them. Stuff with cheese and roll in flour to coat. Coat chiles with Batter and fry over medium heat, turning once. Remove chiles and drain on paper towels. Keep hot in preheated oven.
To serve, divide Hot Sauce among 8 serving plates. Place chiles in center of plate, on top of sauce. Serve with hot tortillas. Chiles also may be stuffed with shredded flank steak fried with onion and chiles or with mincemeat.
Prepare the sauce: Put tomatillos in a saucepan with quartered onion, 6 garlic cloves and chiles. Cover with hot water and bring to a boil. Cook for 25 minutes over low heat. Cool and drain, reserving cooking water. In a blender or food processor, blend these ingredients, adding chopped onion and garlic cloves. Heat oil in a saucepan and fry sauce briefly. Reduce heat and simmer for 30 minutes, seasoning to taste. If sauce thickens too much, add small amount of cooking water. Keep warm.
Prepare the batter: Heat oil in a saucepan. Beat egg whites until soft peaks form. Season with salt. In another bowl, lightly beat egg yolks with a bit of flour. Fold into whites.