Step 1: Roasting the Peppers

1. Heat a griddle or skillet over medium-high heat. Place the poblano peppers directly on the heat and cook for about 15-20 minutes, turning them every few minutes until all sides are charred. The skin should be blistered and blackened.
2. Once fully charred, transfer the peppers to a plastic bag and seal it. Let them rest for about 15-20 minutes. This step helps loosen the skin, making it easier to peel.

Step 2: Preparing the Peppers

1. Remove the peppers from the bag and gently peel off the charred skin. It should come off easily, but if needed, use a paper towel to rub off any stubborn bits. Avoid rinsing the peppers, as this can wash away their smoky flavor.
2. Using a small knife, make a vertical slit down one side of each pepper, from the stem to the tip.
3. Carefully remove the seeds with a spoon or your fingers. If handling seeds directly, consider using gloves to prevent irritation.

Step 3: Preparing the Filling

1. In a medium bowl, combine the crumbled queso fresco, shredded mozzarella, and diced onions.
2. If using chicken or beef, mix it into the cheese filling for a heartier version.
3. Gently stuff each pepper with about ¼ cup of filling. Do not overfill, as this may cause the peppers to break during frying.
4. Press the slit edges together to close the peppers. If needed, secure them with toothpicks to prevent the filling from spilling out.

Step 4: Preparing the Breading and Batter

1. In a shallow dish, mix the flour, salt, pepper, oregano, and garlic powder. Set aside.
2. In a separate large bowl, beat the egg whites using a hand mixer or whisk until soft peaks form. This step is crucial for creating a light, fluffy batter.
3. Add the egg yolks to the whipped whites and gently fold them in until just combined. Avoid over-mixing, as this can deflate the batter.

Step 5: Coating and Frying the Peppers

1. Heat about 1.5 inches of oil in a large skillet over medium-high heat. The oil should reach about 350°F before frying.
2. Roll each stuffed pepper in the flour mixture, ensuring an even coating. This helps the egg batter adhere better.
3. Dip the floured pepper into the egg batter, making sure it is fully coated. Hold the pepper closed while dipping to prevent the filling from escaping.
4. Carefully place the battered pepper into the hot oil. Fry in batches of three to avoid overcrowding the pan.
5. Cook each side for about 3 minutes, or until golden brown and crispy. Use tongs to gently turn the peppers.
6. Transfer the fried peppers to a paper towel-lined plate to drain excess oil.