Turn a gas burner on high. Char the chiles on the burner grate, turning with tongs (Marcela is a pro and uses her hands!), until blackened all over. Or char the chiles under the broiler.
Let them soften.
Transfer the charred chiles to a heavy-duty resealable plastic bag and close. Let stand 10 minutes. The chiles will steam in the bag, making them soft and easy to peel.
Remove the skin.
Gently rub the chiles with paper towels to remove as much skin as possible. It's okay if a few flecks remain-they'll add flavor, so don't rinse them off.
Open the chiles.
Using a paring knife, make a slit across the top of a chile just below the stem, leaving the stem intact. Starting from the middle of the slit, slice lengthwise down to the tip of the pepper (cut through only one layer). Open the chile like a book and pull out the seeds and inner membranes. You may need to use a paring knife to loosen the top of the seedpod. Repeat with the remaining chiles.
Prepare the sauce.
Puree the tomatoes, onion and garlic in a blender until smooth. Warm the olive oil in a saucepan over medium heat. Add the tomato puree and simmer 5 minutes, stirring occasionally; the sauce will be broth-y. Season with salt and pepper and keep warm.
Make the filling.
Place the cheese in a bowl, then add the oregano, crumbling and rubbing it with your fingers to release its flavor. Season the mixture with salt and pepper.
Stuff the chiles.
Fill each chile with about 1/4 cup cheese mixture. Fold in the sides to cover the filling, then thread 2 toothpicks across the seam to form an X. You will probably need to make a second toothpick X to secure each chile so the filling doesn't leak out when you fry.
Mix the batter.
Beat the egg whites with a mixer on high speed until soft peaks form. Add the egg yolk and beat 3 more minutes.
Batter and fry.
Heat about 1 inch vegetable oil in a deep skillet over medium-high heat until a deep-fry thermometer registers 375 degrees F.
Dredge: Pour flour into a shallow dish and season with salt and pepper. One at a time, coat the stuffed chiles with the flour.
Batter: Holding each chile by the stem, lower it into the egg batter to cover completely. Let any excess batter drip off.
Fry: Add the chiles to the hot oil, 1 or 2 at a time; fry, flipping once with tongs, until golden, 1 to 2 minutes per side. Drain on paper towels.
Place a fried chile on each plate and pour the warm sauce over it. Serve immediately.
Photograph by David Tsay
Recipe courtesy Marcela Valladolid for Food Network Magazine