Preheat your broiler to high, or a gas grill to medium-high. Place the poblano peppers on a baking sheet or directly on the grill grates. Roast, turning occasionally with tongs, until the skin is completely charred and blistered on all sides (15-20 minutes).

Immediately transfer the hot peppers to a large plastic bag, seal it, or place them in a bowl and cover tightly with plastic wrap. Let them steam for 10-15 minutes; this helps loosen the skin.

Once cooled enough to handle, remove the peppers from the bag. Under cool running water, or using a dry paper towel, gently peel off the charred skin. Make a lengthwise slit down one side of each pepper and carefully remove the seeds and veins, being careful to keep the stem end intact and the pepper mostly whole. Set aside.

While the peppers are steaming, prepare the sauce. In a blender, combine the quartered tomatoes, chopped white onion, garlic clove, and serrano pepper (if using). Blend until completely smooth. If needed, add a tablespoon or two of water to help it blend.

Heat 1 tablespoon of olive oil in a large skillet or Dutch oven over medium heat. Pour in the blended tomato mixture. Bring to a simmer, then add the chicken broth and dried oregano. Season with salt and black pepper to taste. Reduce heat to low and let the sauce simmer gently for 20-25 minutes, stirring occasionally, allowing the flavors to meld and the sauce to thicken slightly. Keep warm.

In a small bowl, combine the shredded Oaxaca cheese, finely chopped onion (if using), and 1/4 tsp salt. Mix well.

Carefully stuff each peeled poblano pepper with a generous amount of the cheese mixture. Don't overstuff, as the cheese will melt. Gently press the edges of the slit together to enclose the filling.

Place the 1/4 cup of extra all-purpose flour on a plate. Lightly dust each stuffed poblano pepper with flour, ensuring it's completely coated, especially around the slit where it closes. This helps the batter adhere.

In a large, clean mixing bowl, whisk the egg whites with a pinch of salt until stiff peaks form.

In a separate small bowl, whisk the egg yolks with the 2 tablespoons of all-purpose flour until well combined.

Gently fold the egg yolk mixture into the stiff egg whites until just combined, being careful not to deflate the whites too much. The batter should be light and airy.

Heat 3-4 cups of vegetable oil in a large pot or deep fryer to 350-375°F (175-190°C). The oil should be deep enough to partially submerge the chiles.

Working one at a time, carefully dip a flour-dusted, stuffed poblano into the egg batter, ensuring it's fully coated. Gently transfer the battered chile to the hot oil. Fry for 2-3 minutes per side, or until the batter is golden brown and puffed. The cheese inside should be melted.

Using a slotted spoon or spider, carefully remove the fried chile and place it on a plate lined with paper towels to drain excess oil. Repeat with the remaining chiles.

To serve, place one or two Chiles Rellenos on a plate. Generously ladle the warm tomato sauce over them. Garnish with fresh chopped cilantro and a drizzle of Mexican crema or sour cream, if desired. Serve immediately with rice and beans.