SAUCE: Fry minced onion and garlic lightly until tender. Add tomatoes and chicken stock. Bring to boil and season with salt and pepper. Add oregano, rubbed between palms of hands. Boil 1-2 minutes and set aside.
RELLENOS: Cut Monterey Jack cheese into 1"x4"x 1/4" strips with a point at one end. Place cheese into chiles. Place 1 c. flour on a flat plate and roll each chile in flour. Place on dry plate.
BATTER: Allow 1 egg and 1 Tbsp. flour for each 2 chiles. Beat whites of eggs until stiff, then fold in lightly beaten yolks and then 3 Tbsp. flour. Heat oil to moderately hot.
PREPPING/COOKING: Drop stuffed and floured chiles into the batter one at a time. Pick up each with a large spoon, transfer to a saucer, then slide from saucer into hot oil in frying pan. Baste with hot oil until golden brown on each side. Work quickly. Drain them on paper towels and let stand. Don't worry if the nice puffy coating deflates. It will puff up when heated in the thin sauce.
SERVING: Heat sauce to boiling, place chiles into sauce for 3-4 minutes. To serve, place 1/2 cup of sauce on plate under relleno, then cover with 3-4 tablespoons of remaining sauce. Serve with rice, refried beans, and small salad.