1. Roast poblano peppers over a gas flame, turning frequently with tongs, until charred on all sides, about 5 minutes each.  Place in a plastic bag, set aside to allow steaming for 10 minutes.  Rub and remove the pepper charred skin scraping with the dull side of a knife, your hands or a paper towel. Cut peppers open, remove seeds and veins.  Do not remove stem.
2. Stuff about ¾ cup shredded cheese and 3-4 onion strips in each pepper.  Pin closed with 1-2 toothpicks if necessary.
3. Separate egg whites and egg yolks. Place yolks in medium bowl and place egg whites in a large mixing bowl.  Add salt to egg whites.  Beat egg whites with electric mixer on medium speed until stiff peaks form.  Beat yolks with a fork and fold into egg whites until well incorporated.
4. Heat 1 cup oil in a medium frying pan over medium heat.
5. Place flour in a shallow bowl. One at a time, roll each pepper in flour, covering all sides.  Gently shake excess flour. Dip into egg batter holding by the stem. Place in hot oil and carefully baste with spoonfuls of hot oil until golden brown.  Remove from oil and place on a paper-towel-lined plate to soak up excess oil.  Repeat with remaining peppers.
6. To serve: Divide tomato caldillo between 4 shallow plates.  Top with onion slices. Place chile rellenos on tomato caldillo and onion slices.  Garnish with oregano leaves and a sprinkle of cotija cheese.

Directions for Tomato Sauce

1. Heat 2 tablespoons oil in large pan over medium heat, add tomato halves, onion strips and garlic.  Cook for 10 minutes turning tomato halves, onion strips and garlic frequently with tongs; do not burn. Tomatoes should be soft and skins should begin to peel. 
2. Add sautéed tomatoes, onion and garlic to blender container.  Add 2 cups water and blend until smooth.  Transfer blended mixture back to pan and heat over high heat, season with salt and oregano, bring to a boil and reduce heat to medium.  Simmer for 5 minutes. Set aside and keep warm.