Sauté onions in oil. When wilted, add tomatoes and cook until soft, on medium heat, 4 or 5 minutes. Add flour and mix well. Add 1/2 cup chicken broth, bring back to a simmer. Add garlic, oregano, and salt and pepper to taste. Cook until thick, but tomatoes still have chunky texture and garlic and onions are translucent. Set aside until ready to serve rellenos.
Char or roast the peppers on all sides. Once blackened, place in covered bowl or paper bag and let sweat until skins remove easily, about 20 to 30 minutes. Remove blackened skin with a paper towel or your fingers. Carefully make a slit down the center and remove the seeds. Stuff with cheese and use toothpicks to hold them together. Do not over-stuff.
Heat oil in large frying or saute pan.Beat the egg whites until stiff, but not dry, in a bowl. Fold in the flour and egg yolks until incorporated. Dip the stuffed peppers in the batter & coat all sides. Place in hot oil & fry until golden, turn and repeat process until all sides are browned (Pasillas sometimes have three sides). Place on sheet pan lined with paper towels to drain, and keep warm in 200°oven while cooking remaining peppers. When all are cooked, put warm sauce down on warm plates and top with fried peppers, serve extra sauce on the side.
Chef's Note: Roasting the peppers and removing their skins can be done head of time, and so can the sauce, making this a little easier at dinnertime.
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