Lighty rub the chiles with a tiny bit of neutral oil and place on a rimmed baking sheet. Arrange your oven rack the highest it can go. Place the peppers in the oven and broil at 550°F for 4-8 minutes, flip, then roast for another 4-8 minutes. You want the peppers blistered, but not completely burnt. Remove the chiles from the oven and place in a heat proof container. Cover and let sweat for 10-15 minutes or until cool to the touch. Gently rub/slide off the skins.
Make a small slit and deseed if desired. Save one pepper for the ranchero sauce and stuff the remaining peppers by gently placing the cheese inside. Use toothpicks to seal them.
Combine the fire roasted tomatoes, chicken stock, oregano, cumin, and 1 of the roasted green peppers in a blender and blend until smooth.
Pour the sauce into a sauce pan and simmer over low heat, stirring occasionally, until the sauce has deepened in color and flavor. Keep warm while prepping and cooking the chile rellenos.
Make the batter: Separate the eggs into two bowls. Use a mixer to beat the whites into stiff peaks. Whisk the yolks then gently fold them into the egg whites.
Heat 1-2 inches of oil a deep, heavy bottomed skillet over medium hight heat to 350°F. If you don’t have a thermometer, make sure the oil is hot by adding a small dot of egg batter. If it sizzles and floats to the top, it’s hot.
Place the flour in a shallow bowl and gently coat the stuffed peppers, making sure they are completely floured. The flour will help the egg batter stick to the pepper.
Gently dip the floured pepper into the egg batter and immediately place into the hot oil. Repeat with another pepper. Fry for 1-2 minutes per side, or until golden and crisp, flipping once. Remove from the oil when golden and let drain on wire rack or paper towels. Repeat with the remaining peppers, cooking them in batches so the oil doesn’t loose too much heat.
To serve, spoon some warm ranchero sauce onto a plate and top with the crispy relleno. Finish with chopped cilantro and crumbled queso fresco if desired. Enjoy!