Sauce: Put the tomatoes, garlic and onion in a blender. Blend until smooth. In a medium saute pan, heat the oil over medium heat. Add the sauce and simmer for 5 minutes. Season with salt and pepper, to taste.
Chile rellenos: Mix the cheese and oregano in a small bowl. Cut a slit through 1 side of the charred chiles and fill each chile with 1/4 cup of the cheese mixture. Close with toothpicks to hold the filling in place. In a medium bowl, using an electric hand mixer, beat the egg whites on high speed until soft peaks form. Add the egg yolk and continue to beat for about 1 minute.
In a large heavy-bottomed saucepan, pour in enough oil to fill the pan about a third of the way. Heat over medium heat until a deep-frying thermometer inserted in the oil reaches 375 degrees F. (If you don't have a thermometer a cube of bread will brown in about 3 minutes.) Dredge the filled chiles in flour until fully covered. Shake off any excess flour, then dip the chiles into the egg mixture, until well coated. Fry until golden brown and drain on paper towels.
Arrange the chiles on a serving platter, spoon the sauce on top and serve.
To char the chiles (or any fresh chile): Put the chiles over a gas flame or underneath the broiler and cook until they are blackened on all sides. Enclose them in a plastic bag and let stand for 10 minutes to steam, which will make them easier to peel. Peel, stem, and seed the chiles. Once peeled, the chiles may be chopped, sliced or stuffed.